A little bit ago, I did put up a pretty good (I thought) review of the N1 vs. HKS Hiper. But I'll go over the two breifly <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>
well, I've had the N1 and now I'm using the (NA)Hiper. design is almost identical besides 90mm vs. 60mm, except the Hiper is two-piece instead of the onepiece N1. The Hiper's muffler is a tad longer then N1, but slightly smaller diameter. Other then those 2 things....pretty much the same looks, angles and all.
as far as sound, I think the N1 without the silencer is the best sounding of the two, gives a great deep reverberating growl at wot, although quite loud. With the silencer in, much quieter, like half at least, but because of the baffle, you might get a little raspyness...not much, but I noticed a little...
the HKS is much quieter, but the inside of the cabin is still pretty loud at wot. Also, I don't think it sounds as good as the N1, although, it is a nice deep sound, it just doesn't reverberate as well.
ok, maybe reverberate isn't a correct word, go ahead and make fun at me, but you do know what I'm talking about right? and I'm not sure the correct wording...but it's the pulsating sound.
if someone wants to see a N1 on a s13, let me know...I'll put a pic on my website. Also, like I said, the Hiper looks pretty much the same, besides the smaller diameter, so if you want to see the Hiper, let me know too. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>
performance-wise: N1 better topend, lowend suffers a little. HKS Hiper, top-end suffers a little(barely noticable), but you can definately see an improvement on the lowend, so I prefer the Hiper. Of course, this is a butt dyno, so again, I doubt my accuracy between the two..so....basically, without a dyno, it'd be hard to tell. Supposedly, the turbo N1 is not as good as an NA exhaust for a NA KA...supposedly.
anyways, hopes this helps a little.