The rotrex doesn't make any low down power over stock, and adds little to the mid range compared to a turbo or supercharger. However, if you keep the rpm high, they work ok.
I did back to back comparisons with two similar Civics a couple of years ago, both with K20 engines - one with a M62, the other with the rotrex. The M62 made 285 hp, the rotrex 335 hp. Doing time to rpm tests the M62 was faster below 4000 rpm, and then equal to about 6500 rpm or so. Above 7000 rpm the rotrex was faster. Distance to speed from a rolling start gave about the same terminal speed (I think it was 78 mph for the rotrex and 77 mph for the M62). Again, on a race track the rotrex would be much faster because you're able to keep the rpm up.
Bang for buck I'm not sure my $1800 (brand new) K24 + $1000 M62 can be beat...
2005 Elise with Honda engine