I recently bought a three pack of advance quad tap ballasts on eBay. They are tapped for 120,208,240, and 277. I've heard that you could do this on here, but I need to verify it. Could I use the other taps as a step up transformer if I use the 120 tap and could I have it as a step down transformer if I used the 277 tap with 277v?

Yes, the maximum load in VA is then the nominal ballast wattage.
I would guess quite good thing for the 240V or similar experiments...

Wait a minute here..

I take a 120/277/347volt HPS or MH ballast and I simply wire (for Canada 120V) the 120V/COM & ground the ballast core.

Then from that I use the 347V tap with the COM to power another HID ballast that only has a 347volt tap? Like the SOX 135w ballast pictured? That doesn't sound right.