I just had the HKS exhaust installed by my Lexus dealership on Thursday. First off... it looks frickin' awesome! Second... I will admit the different exhaust note does take a bit to get used to, but it a great sound that doesn't sacrifice the quiet sound-level in normal driving. If you drive around under 2500 rpm's... the car sounds pretty much the same. But, if you get into it and the revs go above 2500 rpm's... you get a great aggressive but smooth low V-6 exhaust note. As for hp gains? I didn't buy it for additional hp... so I can't answer that question. I would guess between 5 and 10 hp.
The exhaust is pricey, but it is definitely a great mod!
And best of all... it's covered under the regulary warranty since it was purchased and installed by the dealership.
Couldn't have said it better myself. I had mine installed about a month ago for the exact some reasons... better exhaust note and better looking tail pipes. I have also had the exact same experience with the exhaust note/interior noise. Under normal driving (i.e. under 2,500 rpms - cruising on the interstate) it does not increase the interior noise level at all. And, you gotta love the regular warranty coverage it's pretty great too!