Tought to say, but I'd guess 5-10 hp.
As far as exhaust diameter, a basic rule of thumb is 2 inch for the first 100 hp, then add .5 inch diameter for every 100 hp after that. So it goes like this:
100 hp 2 inch
200 hp 2.5 inch
300 hp 3 inch
400 hp 3.5 inch
This is a very basic rule...obviously there's a lot more to exhaust system design than diameter, but just thought I'd throw that out there.
You can see most S1, S2 cars are putting out less than 300 hp, so they are fine with 2.5 inch (Borla exhaust).
At 350 hp, I'd definitly go 3 inch.
ALso keep in mind, if your making 310-320 hp with a 2.5 inch exhaust, the gains by going over to a 3 inch exhaust may not be that much. Gains are small at first, then go up the more hp you make.