If you want a 2.5" collector and good ground clearance, go for the A'PEXi GT header for the '98spec JDM ITR. It's a 4-2-1 design so it keeps itself up from the ground more than the 4-1 designs.

You'll also need a 2.5" cat and 2.5" exhaust to get the full benefit of the header. Kteller sells what they call a "bolt up piping kit" which is essentially a set of pipes that you bolt on and add your choice of muffler (which they also sell).

If you're not modifying the engine internally, then the gains you see from a 2.5" system over a regular bolt-on header and a regular aftermarket exhuast (most are 60 mm piping which is like 2.36") will be maybe 3-5 hp tops. ITR motors and GSR motors with cams respond better to the 2.5" exhaust than a stock GSR motor.