The cycling between modes is done by a quick on/off, although I'm not sure how may seconds you get on that particular model. Many users tend to prefer a clickie switch for multimodes, so they can do multiple quick presses to cycle up or down through their levels, but I personally still prefer a twisty, as on your G2s.
As for heat, it's not the lexan window you need to worry about, it's the LED itself. When it's on high, and surrounded by the nitrolon body of the G2, the heat can't dissipate as well as it could with a metal head, so the drop-in itself can get too hot. This doesn't have an immediately obvious effect, more of a long term LED lifespan effect. Actually I suppose if it gets hot enough it would instantly burn out, but I have no idea on the facts and figures on that.
You should ideally also get 2 metal heads for the G2, easily found on the Marketplace, then you can have whatever dropin you want running high and bright.