The SR20DET has the stock BOV fitted right next to the intercooler inside the left inner fender. If you plan to fit on an aftermarket BOV, you're better off getting one that you fit on the topside of the engine, instead of a secluded place like the inner fender. If you plan to do that, you'll need to take off the stock BOV and cork the holes up.
AFAIK BOVs has no effect on fuel efficiency, but it will have an effect on engine response. I found that no BOV seems to have the best response. There is almost no lag, since the pressurized air is kept pressurized inside the piping. The only downside to this is that if you plan to raise the boost on stock S13/S14 turbines, no BOV may ruin your turbines faster, since the pressurized air will make its way back into the turbines during gear change. You'll get to hear a cool sound though.