You can get lighter clutch shoes and tigher springs (Mugen 1.1). Some people shave the ends of the shoes or drill holes in the ends to lighten them. The idea that you want the engine to reach higher RPMs before the clutch engages. This will improve bottm end but you sacrifice your smooth powerband because turn the engine into more of an on/off switch. The C5 is notorious for this behavior after you adjust the clutch.

The next thing you can do is to try and richen up the LSN. A lot of people don't know that good bottom end performace on the S7/WS7/WS7II is possible with a pretty rich LSN. I read someplace that one racer lets his RB engine idle for only 10 seconds before it loads up and dies -- that's pretty rich. The OS engines work differently. For this reason, the RB's don't get tuned properly and people have a hard time getting good bottom end performance out of them.

I have to warn you that all this information is second hand as I don't own an RB, but just read stuff off the RB Forums. That's probably the best source for that type of info.

-RobH