Yea, filling the mag when the mag is cold makes a good difference. I also like to put a hand warmer pack on my propane tank to help as well, since it's so darn cold here. Another thing that helps with gas efficiency is mag lip shimming.
Read the following link on how to shim mags:
http://www.airsoftforum.com/board/WI...#entry18271499
This made a HUGE difference in my TM G17.