I didn't go w/ kooks mids I got JBA 1-3/4" long tubes instead...price was also a factor for me and I already had a custom pieced together catback so I didn't want to have to hack that up and retrofit kooks LT's + connection..and the JBA's don't hang low at all as theyr'e not as "long".
I can tell you on my setup (mods in sig magnaflows) - with NO cats, it is louder from outside the car on cold startup but sounds good, at partial throttle (like 1/3 cruising style) it is no louder in the cabin if not quieter since I had AFM tuned out. At WOT it sounds meaner/better - louder? yeah I guess, but not terrible.
You will hear the pinging of the headers but it subides some as they warm up and is less noticeable from inside the cabin.
Get them you won't be disappointed. As far as HP increase...by the way guys talk on this forum LT's get you 40whp....from my experience this is simply not the case. On the dyno I got my mods tuned on stock G8 GT's put down between 280-290whp. After the mods in sig I'm at 344hp/356tq (mustang dyno). A dynojet would probably read 10hp higher at least. It is the cumulative effect of the mods you will notice, it isn't like headers are a 30-40hp boost in themselves, they need the intake/tune/catback to see full results.
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned