Can dual 550 12T motors be used as replacements for the stock 550 21T motors on a 3903 E-Maxx?
They can be used, and there will be a difference between them and stock, but frankly i am not sure how much of one.
With great brushless comes great responsibility.
12T motors will turn much faster with less torque, so you will sense a difference.
2wd Slash VXL
sounds like a re gear would be in order
What gearing set up would you guys recommend for this?
None. The 12T motors are designed to run on a maximum of 8.4V (nominal) but are much happier and last much longer in 7.2V (nominal) applications. The EXV-2 outputs 14.4V (nominal) to each motor. The 12T motors almost certainly won't last very long on that voltage.
I'd heed the warning.NOTE: Titan 12T motors are intended for use in single motor and single battery pack applications, up to 8.4 volts. Traxxas does not recommend the use of Titan 12T motors in dual motor vehicles, such as the E-Maxx, Villain, or Villain EX.
If you're gonna be stupid, you'd better be tough.
Word is bond on the warning.
My local hobby shop just got in a few of the correct titan 550 21T's so I'm good for the week!