I would look at the following websites. I think they have tuning parts and upgrades for the Lexus GS:
http://www.suprastore.com/index.html
http://www.rodmillenstore.com/home/home.htm
They only use the V8 engine here in Japan on the LS and SC. I know Blitz makes Superchargers for it. I have seen them at autoshows over here.