So, your mountain bike doesn't brake well going downhill on roads, and
your solution is to get another mountain bike? For riding on roads I'd
have to recommend, let's see, oh yeah, a road bike.
I'm talking on going down a very steep hill on a small pavement stretch
nearly at the end of a XC trail. Right at the bottom of the hill, the trail
goes back to the woods while the road continues staight. I'm not convinced
that a road bike would brake any better at around 30 mph still on the slope.
Anyway, I find road bikes boring. I prefer the woods to the bike path.