When I contact Schumacher about the limits on the LPF approx 12 months ago, I was informed the 2S Lipo limit was 6.5R. The Schumacher website now states 5.5R limit on Lipo, so maybe they've updated some of the components.
If you are running 5.5R in 4wd I think you are pushing the speedo to it's limit...I've seen the LPF run with a 5.5 in 2wd on Grass without any problem though. I run Pro's in both 2wd and 4wd so I don't have first hand experience of the LPF.
I'm not sure what you mean about the normal GT2.0 - there is the LPF and Pro only -
http://www.speedpassion.net/eu/catagory.asp?c=esc&sc=16
They are the same physical size but the pro is rated at almost twice the Amps...