Splinter Cell 1 fixed that problem with the mouse wheel, that would control your sneaking speed. But dunno if that would even fit here.
Even with the controller the car has very little "in between" speeds. I'm happy you like it thou, I've replayed the game a bit and got BIT better with the car. But I still rather play without it. 
Well, I'm OK with Batmobile (and gliding as a matter of fact) because I switch to controller to do those things when I have to. Most of the time I simply use the "Tank" mode cause it's easier to control.
I hate the controller and use it only when I have to. Unfortunately most games nowadays is developed for the consoles and controller and then ported (poorly) to PC instead of being developed for PC from the very start or at least separately at the same time.
I just hope that BW won't repeat the fail with PC controls from DA:I this time around and make new ME game playable.
Good thing is that Frostbite is a shooter engine so maybe they won't break it completely...