IMHO, I think CES are making overly exaggerated claims, esp for a Skyline.
S15's have tiny exhausts so they can maybe make that claim, but skyline stock exhausts arent as small, they are about 2,5" (R34 GTT anyways), so to make a claim of 40rwkw on a Skyline is pretty out there.
Furthermore, do you think that CES really put their systems through proper flow and backpressure tests, I would imagine that just like any other exhaust shop, they just simply make an exhaust, test that it simply to ensure that it works okay, and then make design templates. Note: they use Magnaflow mufflers and do not design their own so other than the piping, I dont think theres anything overly special to warrant the high price.
Get a BOS importing split dump, hi flow cat, and japanese cat back and you wont regret it.
The only reason why I recommend this is because of the sound factor, japs catbacks "usually" have a better sound. Go for a Kakimoto or Fujitsubo and you wont regret it. Note: if you want something quiet, get an oval muffler, do not get a cannon.
Also, keep in mind that mild steel is much cheaper and easier to repair in the event that you bend the piping or something, mild steel will be harder and hence cost more for you to repair. From what I hear, they also dont make the usual tingling sound.