Whether you class it as "slow" depends on what you're comparing it too really. I wouldn't call a standard IS200 "fast", but I certainly wouldn't call it "sluggish" either. I agree that the looks of the IS200 are amazing, it's very nice inside, lots of extras, handles well, is very smooth and is easy to look after. The engine seems more to have been designed to be a smooth cruiser rather than being a rocket ship.
You may find a few supercharged cars second-hand, but you need to check if they've been looked after properly (e.g. there is a very nice supercharged car on sale from a club member at the moment !). Lexus used to sell a supercharger kit but it was very expensive. They come up from time to time second-hand for around £2k, and are very easy to fit (the kit was designed as an easy add-on).