For the IS300 nope. Probably the best single, reasonably priced upgrade is a new exhaust manifold. But even then you need a solution to stop the engine check light from coming on with the removal of the precats.
I've spent a fortune on my IS300 so far, and the gains are relatively small. I only did them because I wanted a unique UK 300 and they will eventually compliment a F/I setup when I can afford it. To give you an idea:
Performance Exhaust - ~5BHP
£200 - £400 w/ £40 fitting.
Decat performance manifold - ~10BHP (bit more with whole new exhaust system)
£300 - 400 w/ £100 fitting.
Performance Intake - ~8BHP
£200 - £250
Adjustable Cam gear - ~2BHP (but improves power band slightly)
£100 - £150 w/ £50 fitting.
Replacement cam shafts - ~20-25BHP
£500 - £600 w/ £500+ fitting.
With the IS300 its easier to make the car go faster by loosing weight than it is to make it go faster by increasing torque/bhp. Get lighter wheels, ditch the spare wheel, replace the sun roof with a plastic insert, aluminium driveshaft, bucket seats, carbon fiber bonnet and boot...etc. But then its not a Lexus anymore.
EDIT: Also if your priority is to make the IS300 more fun to drive, then I can suggest the accelerator pedal spring mod and also the throttle cable mod. They both make the car more responsive to what your foot is doing and make the car feel like there is more power on tap.