contact these folks:
http://www.jpimportz.org/
very respected on the clublexus.com forums. twin turbos, supercharging, stroker kits... you name it :)
Hi There,
I also have a SWEET 1999 GS400 ! I love it. A friend of mine had his chipped, and he beat a Ferrari Testarossa in a "race" on the freeway late at night. As far as the chip goes, I don't have an answer yet, but it can be done without harming the engine.