for a basically stock motor, go with a comptech 4-2-1, but DC JDM 4-1 will flow better and JDM 4-1 is pretty good also.

and for exhaust, comptech also makes a good exhaust, but greddy sp, dc sports, apexi ws, and RSR(if u can find them) are all pretty quiet and dont have that big canister muffler. or u can go sleeper look and use a ITR exhaust, which is already 2.25" piping it will flow better than your stock ls exhaust and no noise.