The UK Supra weighs more than a IS200 or 300. It's a big car. The IS300 uses the Supra non-turbo engine so it is partly designed for the engine.
A UK Supra had electric windows, mirrors, CD player, electric seats, climate control, headlamp washers, power steering, 6-speed manual or 4 speed auto, remote central locking, CAT1 alarm. In fact the one thing it is missing is the sunroof and that's not there to improve handling, although your can get an import targa roof model. So I don't think you can say it doesn't have all the luxury, in fact the IS200 doesn't have leather interior which the Supra has.
There are several Supras with 1000+bhp so trying to create the fastest one would not be a easy task plus it's all been done before, how many IS200's do you know with 700bhp?