part of my problem w/ kits is that too many people tend to get them too soon. by that, i mean people who aren't used to riding mopeds and/or have bikes that they recently acquired.
i think the best course is to have a moped and slowly work the kinks out of it in stock condition. i would like to be sure the engine is fine, the flywheel is working well, and that i've gotten all the performance i can get from it ... stock. then, if i want more performance, i can start working on regular mods ... and only then ... after some time troubleshooting the larger carb/muffler combo ... then go for a speed kit.
but i'm still leery of speed kits. mopeds were built to specifications for 50cc and 1-2 hp. some moped frames might not withstand the stress tolerances of more hp. also, some brakes might not hold up when braking from 50+ mph.