I have seen the US guys running 272 Brian Crower cams (replacing both) which have been designed to keep the VVTi, I know that having installed these they also uprated the valve springs and some increase the clearance by 1mm too, which gained them 50hp on a turbo'd engine so not sure the outcome on a normally aspirated.
With that they installed adjustable cam gear, I believe you can only get a exhaust side cam gear on the IS300, using 2nd generation HKS as the 1st gen had problems, but more preferably using Unorthodox or Titan gears.
I would think about what you want to gain, if you are after more power to get to say 300bhp you'll need to do cams, headwork, exhaust, ecu and for the cost you would probably be better off putting a turbo on. I have seen a stock IS300,
2JZ-GE engine and internals, running 415hp.
Check also - http://my.is/forums/f88/best-cam-gear-225242/
This below thread is interesting to note as this guy has gone through a list of parts he wants to install with feedback without using forced induction.
http://my.is/forums/f88/my-desired-set-up-...oosting-393153/
Another extract,
You can put a cam gear on without having your car tuned. The only way I’d tune the car is if you had a full exhaust, intake, cams, wide band, and some kind of management. If you have just a cat back an intake you will be fine installing the cam gear and setting to -4. A cam gear will not really yield any power even if you’re tuned, but what it will do is make your power band more consistent. What I mean by that is you will feel your car pull throughout the whole rpm band. even if you tune a cam gear you may only pick up 5 to 6 whp at the most and I’m not going to spend the money on tune if that’s the highest HP I would get!!
Hope I may have been some help, otherwise keep searching the forums above :)