some supra short blocks have been known to hold upwards of 700 to 800 RWHP. There have been some that have hit 900's i believe with a stock TT shortblock.
The IS300 head can "take" all the power you want. If you are asking how well it FLOWS at higher levels of power, thats different. Based on the experiences of many on this site, the stock IS300 head flows 500ish RWHP very well. If you want much more power, lets say 700-800, you should do some headwork for better volumetric efficiency.
Volumetric efficiency= the efficiency of the motor to move air(basic definition). Having a better flowing head will increase your VE. Keep in mind, you will also need a nicely flowing exhaust system. Having a 2.5 inch exhaust with high amounts of power wont do any good. Try 3+ inches. For 650+, try 3.5 inch.