Coupe-This - I am also looking to get one of these kits from nengun, althought I'm looking at either the Greddy V-SPL or Blitz LM.
Depending on the power your looking to pull from your car, the Greddy V-SPL is recommended to comfortably handle 250rwkw any more than that you'd be looking at the high flowing R-SPL. I have heard lots of good news from the Blitz LM kits but my personal opinion I don't really like the use of original piping. Where I think the Greddy uses cross-over piping which you will have to make a hole near the whiper/radiator over flow bottle.
Only reason for me saying this is that the Blitz piping bends 180degrees on the outlet side, and the idea is for a cold air and flowing intercooler system, like an exhaust you want it to flow the hot air out.. you don't see a 180degree bend in any exhaust.