I never clocked it exactly, but somewhere around 16-18 with tall gearing across a gym floor, and the faster it goes the more stable it gets.
Some of the mods I have on it from what I can remember are:
Aluminum wheels with soft compound tires
turbo chip
upgraded ball bearing motor (don't remember what kind but it was like 40 bucks alone)
Aluminum motor heat sink
softer springs in the front and rear
Carbon fiber brackets for rear shocks
1100 ma-hr batteries
Aluminum ball diff
Ball bearings on all moving parts (motor, differential, all wheels, and motor)
stainless kingpins
aluminum steering knuckles
There's probably some more stuff but it's been 3 years since i spent any money on it.
It's in these pics here. (Honda NSX body)