I had a thought experiment: Imagine 4 fluorescent tubes, all of them 1200 mm long and 38 mm diameter.
One of them is a standard F40T12 with about 3 torr of argon. Another tube is filled to 25 torr of argon. A third one is only filled to 0.1 torr of argon and a 4th one only has the mercury vapor pressure in it, no gas filling.

If we connect them to a current source supplying exactly 430 mA of current, am I right to  assume that the tube filled at 25 torr would have the highest arc voltage and the one containing only the mercury vapor pressure the lowest?

I assume this effect is not linear, so tripling the gas filling pressure wouldn't triple the arc voltage.

These experimental lamps wouldn't be practical, but it would proably offer the possibility of some interesting experiments.

I couldn't find any documentation about gas filling presures F30T12 and F30T8 (pure Ar in both). Since a reduced tube diameter increases the voltage, it's expected that the gas filling pressure in F30T8 is significantly lower to keep the arc voltage of the two tubes the same.

Another interesting topic would the the gas filling old Philips 16, 32 and 50 W tubes (pure argon despite being T8), which have significantly higher arc voltage than standard 600, 1200 and 1500 mm long tubes.

Increasing inert buffer gas pressure (within a reason) will just slightly increase arc (burning) voltage. What will happen at pressure decrease is more complicated, voltage will certainly increase again at some point. A tube with no buffer gas won't ignite at all at practical conditions.