In Australia, initially Queensland, we have been running the Novak 13.5 bonded rotor in our 27t stock class for over a year now and have found them to be a good match. We have allowed them at clubs and smaller events. i believe this years Victorian Titles will also allow brushless 13.5 (LRP & Novak only) so that will be the first large event to do so.
Throughout our testing and competition we have discovered that a good tuned 27t brushed motor still provides for faster laps than the bonded brushless equivilant, but in the hands of decent drivers. If you allow the bonded versions only to run with your brushed stock class, you will not see any great leaps forward in performance, however the sintered rotor is way too fast and would defenitely be an advantage.
We have also been using lipo's as well and have found now real dicernable edge in performance on the track apart from a much simpler pit bench.
Hope this helps
Cheers
Aaron