There are a few exhausts systems out there...try greddy, tanabe, HKS, Megan Racing, etc... Add an intake, megan racing headers and you have done about all that is available for reasonable cost. LMS has a supercharger for the IS250 but its not on their website, don't remember the cost but it was expensive, maybe $4500.00 plus installation? Someone is going to do a reprograming module for it soon, maybe Jet or superchips may already have done one by now.
BTW, 2006 IS250 6sp MT Red/Blk 19 months ago and still love it. Mods include JIC Magics Coilovers, EBC Rotors and pads, 5Zigen ZR520 Wheels, Ings Body Kit, VIS Carbon Fiber hood and trunk lid, Greddy Exhaust, AEM Intake, Megan Racing Headers, Custom Sound System, HID conversions for the Headlights and fog lights, more mods to come....