I realized last winter that my 914 was evolving into a weekend AX fun car, and bought a Rennshift before James raised the price $35. The 2-3 plane is soooo solid, and since most AX races are 2nd and 3rd gear for 90% of the course, it has helped immensely. Super confident 3->2 downshifts and 4->3 downshifts and never grind reverse on the 1->2 upshift.
If you're primary cruisin, it might be overkill, but if you like pushing things a bit, it will almost eliminate embarassing (and costly) missed shifts. The bushing comment is right on, gotta have the entire system solid for the Rennshift to work its magic.