Not sure if the RB X12 comes with a 5 port flavor. I'm thinking they probably did, since the RB V12 comes in 3 or 5 port.

I've seen RB V12 5 ports during a race, and they're beasts. It should beat an OS 12 TZ 5 port at all RPM ranges. I have both an OS 12 TZ 5 port and a Mugen X12. The Mugen X12 is an awesome engine. Not many 3 port engines can beat it, and the Mugen X12 is stronger at low and has a bit more top end than my OS 12TZ 5 port.

The RB X12 is a nice engine, but somewhat slow by today's standards. The OS 12TZ 5 port will beat it quite easily. I've raced my friends RB X12. If you have a large track with a straightaway of more than 150ft, the RB X12 will start to lose its power halfway thru the straight. The OS 12TZ 5 port, will eat it up, since it has nice top end power.