knowing that there arent that many mods that will improve my performance significantly, i still did it to gain some performace. and getting the looks and sound was just added bonus. x]if you're looking for mods specifically to make the car faster, don't bother.
No intake for the ISx50 adds more than 3-5 horsepower, and many (mostly the short rams) might actually cost power because they're sucking in hot air. The Joe Z pipe is going to be the best bang/buck as it shows 3-5 hp gains on a dyno and cost about 1/3rd what the usual aftermarket intakes do WITHOUT sucking up hot engine air or being obnoxiously loud.
For exhaust, assuming you want to stay legal, nothing will add more than 5-7 hp and it'll run around $550-600 (for a Tanabe, which is a very nice, well built system that's fairly quiet, but still doesn't add much power).
If you don't mind being illegal (ie removing cats and such) then there's some full exhausts that might net you 10-15 hp but cost will be more in the 1000-1500 range.
Now, if you want your car to be much -louder- then you have plenty of good options for intake/exhaust. Making it much faster, not so much.
Now, if you -really- don't mind being illegal, Daniel Mazzuri is making headers for the car.. they delete the primary cats and they replace the rear O2 sensors with sims... but they're the first mod I've seen for the IS350 that should actually show significant power gains (on a 250 he was showing something like 25-30 more torque, the 350 should be even better)...and the cost is in between the legal and not-legal exhausts that all add a lot less power.
Install is a pain though (at 4-5 hours) so that'll bump the cost a bit if you can't do it yourself... and you'll badly fail any visual inspection...but bang for the buck it's a decent choise if the legal stuff isn't an issue for you.