[Andy Brockway]

You might try taking off one CIM from each side. Since the interest is in top speed, a single motor should provide the answer. Also try the single in both positions, by trial and error (well somewhat scientific) you should be able to narrow it down to which motor(s) or gears are causing the problem. A burr or slight clash from the CIM gear can also reduce your speed.

I know you checked but any interferences?