for horse power you are looking to about a 10whp-15whp gain. Also it helps alot if you have an intake + exhaust. there are 4 flash programs to choose from. you can have your vtec crossover lowered to 5200 and fuel cutoff set at 8600!!!! I've been reading alot about it and want it, but there is a few things holding me back.
1) waiting for warrenty to end
2) waiting to see long term effects in others rsx'a
3) you have to send your ecu, immobilizer and keys to cali ( i live in mass)

but from what i hear its well worth it besides a turbo for the money.

there is alot of info over at clubrsx, just do a search there.