There are H11 HID kits on the market. Some people have had good luck with Mtech. Usually it's best to get the kit with integrated ignitor and ballast because there's not much room to locate them in the engine compartmant. Also, 2007 models can be ordered with HID as a single option.
The auto trans could get better highway mileage if the rear axle ratio is different, thus running fewer RPM per mile.
Sport suspension lowers a little less than 1/2 inch.
Some performance mods are available, mostly intake and exhaust. The cars were totally new just last year and it takes the aftermarket some time to catch up. This has nothing to do with Lexus, as they aren't in the aftermarket mod business.
With an IS350 you're already getting a very well performing car, so unless you just want to make it louder, there's really no compelling reason for trying to "improve".
The previous generation, IS300 was more mod friendly, although there really wasn't much available early on. But since the motor is so similar to the Supra, people figured out how to turbo it to anywhere from 300 to 700 HP and more. Even the most fanatic performance people will tell you that with that car, turbo is really the only way to make significant power.