It's mostly hype, and it's not for amateurs to try installing, more often than not, it frags the 'ped...sometimes even on a professional installation.
The general consensus seems to be that stock is better and more reliable.
You can improve performance quite a bit without resorting to the speedkit, via other, simpler mods that are less risky.
On the other hand, a fully-modded Tomos A5, with every concievable option and flawless tuning, and the 70cc kit, you could technically hit 70Mph, but the question is - are you crazy enough to try ?
Moped brakes are rated for a top of 30mph, even on some 'peds that hit 32-35mph stock (the Golden Bullet, in perfect tune, wound full out, could do that)....and you want to run that puppy up to 70mph and then try to STOP ? good luck.
Add to this, that most 'peds are single chain drive and when the speed starts to exceed factory limitations, it causes the bike to 'crab' (wander/jerk left, cause that's the side the chain is on) which gets progressively worse as the drive chain loosens due to wear.
For most folks, the risk isn't worth it, too much chance of fragging your precious 'ped, and at high speeds, it becomes VERY difficult to control the 'ped, you really have to know what you are doing, or you'll hurt your 'ped and possibly yourself in the process.
I reccommend sticking with the simpler mods, really.
(I may be nutz enough to try, but I been doin this for 15+ years..)