I have Vishnu Stage 0. I want a Greddy Evo catback. What kind of gains can I expect. Is it necessary to get a reflash now? or can I wait until later when I get a downpipe, midpipe? How much louder will a downpipe/midpipe make the exhaust? :)

You can expect enormous gains. It's probably make your wrx into a 10 second car.

You won't need to worry about a reflash from a catback.

wow! a 10 second wrx with just a catback. wait till I tell everyone. :p