I take it you're using 48P gears and not M1? A 90T M1 would be huge!

How close it is to meshing? What wind is your motor? You can gear those motors quite high as they have tons of torque available. Especially for a "small" MT like the Stampede.
I want to use a 15xx in my Rustler but I'm also worried about having gear ratio selections be limited by meshing ability, more so than with 540 size motors. I wouldn't use a finned Neu, but do you have any pictures of your setup?