We have done a bit of comparison testing with 21.5 B/L motors and stock Silvercans. The B/L motors used were the Novak Ballistic and the Reedy Sonic 21.5. The lap times were pretty close with the Silvercan cars with a slight edge. The Silvercan cars were fully sorted and the two B/L cars were not as fully "dialed". In the end, the 21.5 should have a slight edge, but they were close enough that they could probably race together.
Please note that this was at one track and after only one full day of testing. It's probably too early to come to a definite conclusion, but our early testing is very promising. Also the lap timer was set up so that accurate lap times could be measured. These conclusions are based on actual lap times and not comparison running or any other subjective measure of performance.