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Just FYI, I have experienced this problem on my Chinascoot as well.
If you're running out of gas at full bore, the problem most likely isn't the pump.
The problem is lack of vacuum; more specifically, its the cheapass hose clamps that the bike ships with. They are pretty much one-use items, and if you (for instance) take the vacuum hose off of the intake manifold, you need to replace the hose clamp with a new one when you put it back on.
I've also had the "carburetor bowl mysteriously empty after a few days" problems, although that was due to a slow leak on the carburetor drain line caused by, you guessed it, hose clamps. Once I replaced every vacuum and fuel line clamp with clamps from the local True Value hardware store, I no longer had any hard starting or fuel delivery problems (even after the bike was sitting for a month).
As a final note, my YY250T has a hard top speed (on a flat surface) of around 70mph. (I weigh 260 lbs.) The variator doesn't allow the engine to go above 6K rpm (although I can blow by redline on the centerstand), so I suspect it would probably have more oomph if the variator allowed higher RPMs, but I've no inclination to try tuning this bike, since I had a blow-by in the head gasket about 3800 miles into it (with mostly secondary street usage). If you want more speed, you're better off with a second hand Taiwanese or Japanese scooter (which will easily do 80mph with a 250cc carbureted engine) or just get a bigger bike. But yeah, it does sound like you have a fuel problem if you aren't getting over 55. (Make sure you use a GPS, the speedometer is not accurate).
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