Lowe's when they were still around in Canada started discontinuing HID lamps in 2012 or so by cutting down on the range of lamps offered. By 2014, Lowe's Canada pretty much stopped selling HID lamps. I got a few cheap HID lamps at the time when they were clearing out the stock, mostly newer Sylvania lamps.

In late 2023, Lowe's spun off the Canadian division along with the Canadian RONA stores they owned. The Lowe's stores were rebranded under the RONA banner by early 2024. 

Going by reference ballast characteristics (not exactly realistic, but good enough), the 400W HPS needs 38.3 ohms at 220V while the 400W MH needs 45 ohms at 220V. That is a 16% difference, so not insignificant.

According to my Desmos calculator which has a history of being semi-correct, running a 400W MH lamp on 38.3 ohms at 220V will run it at almost 4 amps, which is a 23% lamp overdrive, assuming that the lamp voltage doesn't significantly decrease from the normal 135V (not exactly realistic, but good enough). Very significant lamp overdrive, but probably fine for short periods or with a lamp you don't care about.

This would mean that the ballast meant for ballasting 400W lamps is now ballasting a lamp that is operating at more than 400W (probably around 500W if I were to take a wild guess). The secondary coil would probably be fine since it would be carrying less current than normal, but the primary will probably carry more current than normal due to the increased secondary wattage. If you choose to do this, just monitor the temperature and make sure it doesn't get too hot I guess. Long-term it will certainly reduce ballast lifespan.

Currently, I have a 400W PSMH Westinghouse lamp made in China, and I also have a GE M400 in 400W HPS, which are obviously two different lamp technologies. Recently, I was wondering what would happen if I ran the 400W PSMH lamp on the 400W HPS ballast. The arc voltage drop is higher than that of a 400W HPS lamp, so how would this affect the ballast? Would it overload the ballast?

That's interestin. What function does the carbon dioxide has in these lamps?

Good question!  I am also not entirely certain, but suspect it helps to waste some energy in the form of heat at low current operation, so as to help keep the electrodes warm enough to prevent sputtering.

Yes they indeed are!😀, I have a 2 watt filament lamp at the top of my stairs that runs 24/7, it’s slightly noticeable by eye, but in the photo I have turned the colour saturation up so you can clearly see it👍

Depends on which losses. Mechanical friction within the compressor is related to forces, so part with rpm (inertial forces), partly to pressure difference, partly fixed per pumped volume.
Flow losses use to go quadratic with flow rate, so higher percentage with higher power.
Blow-by losses are related to just pressure differences, so percentage wise become higher at low rpm so low power.
Valve backpressure is percentage wise rather fixed.
Motor winding losses are related to torque, so pressure difference. So percentage wise become higher at lower power.

All this means each machine has obviously its most efficient operating point power level, well adjusted VFD system has that set as the minimum power setting (when the demand is below that, instead of slowing down further it turns into the classic on/off PWM regulation, just using the minimum power level when on),
The ability of VFD to go way faster so boost the power up means the system could be designed with the best efficiency at the power level barely sufficient to maintain the temperatures with the outside condition that are there most of the time. If conditions change so the system needs higher power, it can boost up so is able to keep up, even when the efficiency may actually drop. It is not an issue, because such abnormal condition usually are not present for that long time.
With fixed power system the thing has to be rated for the maximum load, but as most of the time the need is way lower, the system is stuck with that fixed power.

Don't trust what you see in the photos.  They'll use old photos for new lamps. 

On the outside, compurter-controlled ofcourse, but also to have 100% RGB LED's (in some mini-ish size) ... but not the low-voltage ones like you can get now, instead ones that run on standard 120v (the low voltage ones are cool in that you can control every 'bulb' independently, but then you also have to deal with a bunch of 12v power-supplies, limited distances, limiting current(due to running off power supplies), very expensive controllers, etc. 120v on the other hand its simpler controllers, no power supplies, no distance limits, limiting current isn't much of an issue, but also means the whole string will have to change together which I'd be totally fine with.)

I do currently computer-control things (so have that part down), but its all standard mini-type lights (mostly LED), with multiple single-colored strings ran together to give different colors...so end up with a small bundle of strings outlining the house. A single RGB string would be so much cleaner/easier! .lol. but what I want doesn't exist(even the type of LED needed is non-existant)

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Inside, I already have my little of 'forest' of trees (mostly LED, individual color per tree)...really wouldn't change much there other than adding more computer-control, and in particular one that could do dimming. (could easily be done, but I don't wish to spend the money & time it'd take to build controllers)

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I have in the past really wanted to do dimmable fluorescents as part of the outside display (have used fluorescent in the past but couldn't control the dimming). I wouldn't bother anymore (even if I could build an interface to make 0-10v dimmable ballasts 'talk' to computer-controls) fluorescent is just too finicky - particularly about bitter cold weather. If it exists, 120v colored dimmable tubes would be far far better/easier.

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Overall I don't do as much as I used to outside...fact is I'm old & worn out / just can't go climbing around in trees or as much on the roof as I used to. Making things easier/less time-consuming is better.
Oh and.. I actually have tons of sets of (used & new, mostly incandescent, but some LED too) mini lights that I'd like to just sell off because I'll never use them. (Yep even old XmasLightGuy wants to clear out a good part of his stuff!) If anyone here is in CO & needs lights, let me know. .lol.

Another lighting pet peeve that bothers me is when metal halide lamps and other lamps with burning position restrictions are operated in the incorrect burning position. For example, a base up only burn metal halide lamp being operated horizontally.

If I ever have a house, I just want to do a simple setup. Probably some colored lights out front and a few spiral lighted trees, and electric candles in the windows.