There are a few kits available in the US, try looking at any of the following:
Toyomoto http://www.toyomoto.com
Peter Farrell Supercars http://www.pfsupercars.com
TBKO http://www.tbko.com/is300_turbo.html
XS Engineering http://www.i-m-racing.com/comsoonxseni.html
SRT http://www.i-m-racing.com/srtstag1turk.html
There are many more but these are the only ones I can remember at the moment.
I run my HKS dampers quite hard but when changing to the softest setting the car is just as comfortable as before despite the lowering. The only problem I've experienced is hitting the front splitter on the up side of some of the bumpy roads in London.
agreed, the front tyres on my car went very quickly and only on the inside edge (this was before any modifications), the tyres have been fine since the HKS dampers etc. went on
its moody, the car and supercharger are unhappy about the 2JZ-GTE swap coming soon, its just jealous and trying to make your life hard for the next month
I believe there is one member on the board with a powerflow exhaust (haven't seem him for a while though) - I think you are probably better off sticking with something tried and tested though. HKS seem to be pretty proven on the IS.
14,500 miles in nearly 3 years. Need to get my wheels replaced too, I've got 3 weeks left before the car is 3 years old, not sure if lexus will replace them as they've had a bit of a hard life!