What would you do?

I think that the best way for you to get the most power out of a stock exhaust would be to gain high rpm's. To do this you will need to get new camshafts and cam gears (Todas are good) also a new ecu (mugen or b16 ecu) in order to get higher revs. However, high revs are bad if you don't set your car up right. So you might want to consider JDM type r pistons and rods. Also if you have the money and can find a place that will do it have your block re-sleved corectly. This should also raise your compression somewhere in the 11's so you could most likely rev around 8,800. Also you may eventually want a vtec controller this way you can fine tune your vtec to kick at the right rpms. Since the gsr already has a lot of bottom end torque i think that this would be a good way for you to go.

I know that this is a little much but just save up and do it right and your car with be a beast.

Chris