I can't say for sure about the Doppler, but from what most say I would expect that if you are looking for a top end ripper the Doppler would let you turn higher RPM's than the G3, but the G3 has a wider range of power. I know on my bike the G3 for sure lets you keep pedals and I don't think the Doppler does (at least without some modifications). I love my g3, but really would like to take my same set up with a doppler and see the differences. The G3 seems to let a pug hit some pretty fast top speeds.
As far as the G2, I feel like that pipe would be better suited for a 50 cc kit rather than a larger displacement kit. I think that is designed for even higher rpms than the doppler. I read that on some french tuning site, so that isn't from my personal experience. May not be true, but I think you are right Josh, the two best choices are probably the G3 or the Doppler.