main differences....
the C5 came with a different intake manifold
larger Throttle Body
port/polished head from the factory
dual valve springs
more aggressive cams
and of course the ECU to control it all
in the end the power curve is a lot smoother and resulted in more output. You really can't say it's only 15hp, its way more than that in drivability.
read this post for more info from others


I'd covert your rex to OBD1, going to OBD2 from OBD0 would be a huge task. and in the end you'd want 1 to go stand alone anyway.
Since the ITR ECU is OBD2a/b (there was one year it was OBD1 but it nearly imposable to find one of the ECUs) a configurable ECU is about the only way to go. Hondata is great and OEM quality.

the C5 is great for N/A but if you want big numbers and plan to boost, stick with the C1, much cheaper and more turbo friendly CR. Plus if you pop it, you don't cry quite as much.