If you are reffering to the mugen mt12, it is a very good engine. It is a nova rossie built mill, and it is very comparable to the rb. I have expierience with that engine running in an rc10gt, and it was the fastest truck on the track, Mind you it was racing with nova rossis, os cv-r's, and picco powered trucks. I'm not sure if its faster than the rb, but it is very comparable. Either the rb or mt12, you can't go wrong. They are both very high quality very fast engines.
The MT 12 Bighead is the one I ordered for my TC3. Unfortunately, I dont think I will get to drive it this year. Unless someone opens an idoor track somewhere close.