I run a P91 in my G2 with no problems. You should use a glass lens with P91.
(I'm not 100% sure if mine is plastic or not, but short 1 -2 min and it seems to be fine!!!)
Incan in nitrolon is fine...I seem to remember reading originaly G2's had plastic lenzes, then moved to glass.
I think all the G2 lenses are still lexan. Lexan should be okay in short bursts with P91 or IMR-9, but I would think two minutes is probably pushing it.
I have a G3 (pyrex, hex bezel) head on my black G2 and a hardened mineral glass lense in my od-green G2.