We've done some very limited testing with 17.5 vs Sport Tuned Mabuchi on our 60' by 60' track and found the times very close so long as we limited the gear ratio on the BL system. I've been wanting to try 18.5 and 21.5 but none of the guys seem to have them to test with...maybe in the near future. For now the motors are approved for testing only in my race and do not score points in the series.
With the price of brushed ESCs coming down so low now that Tamiya is including them in their M03 kits and BL being approved in some of the TCS classes I don't think it is going to be much longer before we see Tamiya coming out with a Mabuchi BL equivalent and including it with their kits. When this happens this will be a major change in the industry.