I put on a Greddy copy (cost $300) with my stock throttle body when I got my RB30 so no direct before and after comparisons but I am happy with the way my car goes. The main downside, and it appears to apply to the Plazmaman as well, is that the angle of the intake means your piping has to go where the battery is and I had to hack away some of the fusebox to make room for the throttlebody.
I believe some people are developing adaptor plates to make it easier to fit RB26 plenums although if I had more money I would just have put an RB26 head on my RB30.