I'm pulling about 74mph on mine. It has about the same mods you mentioned. I am still running the stock gears.

DWT

 

Here's a link to a gearing calculator for the 400ex. It will give you the top speed for each gear with the sprocket size, max engine rpm, and tire size you select. It starts with stock gearing and tire size, although I believe the rpm is about 250 below the limiter. So a completely stock (by stock I'm only referring to what matters here- sprockets, rev limit, tire size) 400ex will pull 70.7 It will pull 77.71 with 22's