you'll probably never get close to 55+ with a shift kit. it's designed basically for acceleration so you don't have to pedal off the line, but the motors aren't strong enough to pull the bike into the higher gears.
i run a heavily modified GT4 with a 16mm mikuni carb, a modified Harley/Aermacchi head, custom pipe, and a reversed barrel (here's the build thread: http://motorbicycling.com/f38/one-mo...eet-21037.html
i'm also 135lbs with a 28t sprocket.
the bike's been GPS'd at 54mph and will go faster, i just ran outta road
. gonna do some new videos soon.
it takes a little while to get up to speed, but not enough to make it unrideable, i just have to pedal a little more to get it going.
although i've never tried a GT5 super rat yet, the GT4 is, in my opinion, the best grubee motor out there. they were made by a different factory than the new GT5's and the quality is much better. the castings are much smoother, and they seem to run better than every other motor i've seen, ridden, and fixed.
i've built a few of the new GT5's (not the rats) and they're ok, but they're noticeably inferior to the GT4's. the castings on the fins are horrible, and they just don't make as much power as a stock GT4.
i've followed the threads about the super rats, and they seem to have a lot of problems, but without having owned one myself, i dunno if it's the engine or the guy building the bike (no offense to anyone, i just don't know the experience level of the people using them...)
realistically, a well tuned motor with a shift kit should see speeds in the 40-45mph range, but again, it depends on the builder's skills, weight, bike, etc.
if you want to comfortably cruise at 55+, you're going to need to go a different route, like a morini, or hopped up 4 stroke, or get away from a motorized bicycle altogether. it's not impossible with a chinese two stroke, but it's difficult.
also, when heavily modifying the chinese two stroker, you get into a whole 'nother realm of problems, trial and error, and longevity issues.
although there's a lot of hop-up parts on the aftermarket, the true potential for high speed racers hasn't been reached yet.
the 40mph barrier has only recently been breached by the masses, but 50 is still out of reach for the average builder.
as for building a bike that can handle the higher speeds, it's not that difficult, but it does take time, money and knowledge. my bikes aren't just off-the-shelf beach cruisers, they're heavy-duty steel monsters, weighing 70lbs+.
as the old saying goes, "speed's just a question of money. how fast ya wanna go?"