The exhaust will restrict you but it will work for the time being. The standard centre and especially rear silencer are the biggest restrictions. Changine them will help a great deal alone. Go to 2 1/2" straight through with that setup Or anything upto 3" if you want to allow for future mods. Make one out of universal pipe and an overaxle bend from demontweeks for cheap enough
Other worthwhile mods are an electric fan. Grab one from the scrappy if on a budget. A piggyback ecu for the s13 injectors will be needed.
Other than that you've pretty much got it planned out very well. Glad to see your not neglecting the fuel pump
the piper cam doesnt really give you much power for money spent so i wouldnt really be too concerned/botherd about doing that unless your planning on using a t28rs or t3 etc