Watts stay the same and amps change. So if your ballast is rated 1000 watts at 120 volts that is 8.3 Amps (probably this is the 8 amps you saw). Your bulb is 300volts and thus will be somewhat under 3.3 Amps (assuming the ballast takes up some power).
When you compress air it heats, and when you decompress it it cools, but how much?
Working from http://en.wikipedia.org/wiki/Gay-Lussac's_law I find the equation
pressure/temperature=constant
so I derived for example
if the air is 10°C (283.16°K) and the pressure is 14PSI so...
14/283.16 =...