Having used an "out of the box" grubee kit, boygofast kit, and now a fred race motor.... there are MAJOR quality gaps.
grubee kits on a whole are decent quality, some kits have a ****ty carb design - but they are kinda hit or miss with getting a straight crank. friend and i bought 3 kits, 2 from one batch, one from another. the 2 from the same batch were (and mine still is) great. 43.62mph with mods on mine. the 3rd vibrated like a banshee past 4500rpm (ish)
boygofast kits are decent, they seem to be a touch faster top end out of the box than grubee kits, may be from the 40mm stroke vs 38mm on the gt5 grubee. still kinda iffy with straight cranks
and finally, the FREDMOTOR. My buddy has his installed and it did 40 out of the box on a 36t. still hafta dial in the carb, but thats not a bad start. its SUPREMELY smooth running. Sounds great. LOOKS great, head is beautifully machine, EVERY single bolt is replaced with uprated hardware, the clutch is properly lubed and adjusted, everything is perfectly sealed. We did a 92.6 mile trip the day we installed his motor - not a single problem. could be more fuel efficient, but i think he needs to downjet it.
long story short.... if moneys no object, look at a fred motor. i gave my friend **** about buying one until it arrived. needless to say i've got my own fred motor in a box
in the meantime Im having cranks balanced and straightened so i can build my own critters!