.21's can compete with it as far as topend, but the torque that this engine makes is insane. I ran mine with a 2006 pipe and it went through the powerband screaming at the max RPM within 10=15 feet in my LST2, so I cant even fathom what it would be like in a buggy. I personally wouldnt bother with the premodded versions if you are going to get really crazy with it, if you were going to get a mod, I would recommend calling up an engine mod company and discussing what you want from the engine and then letting them taylor it to what you want. The only other engine I could see holding a match to this in all areas is the RB 928, its also a very high quality engine ontop of being as powerful.