I _think_ (i know for sure on obd2a GSR harness) the difference is that the GSR harness, breaks off in the engine bay at the "canon plugs", and the Si Harness, goes all the way into the car to the ecu.
I am selling a 00 Civic Si harness pm if you are interested, i have the p2t ecu also.
(one plug on the harness is damaged, and you will need to add it back on, it's the black 'stick' looking plug that goes to the temperature sensor i think? it's about 2 inches long, single wire plug, it got stuck and i pulled too hard. You can get the plug off from almost any honda harness)
Basically more work than needed to get it to go.
You should look around for an Ex (might need extra wires added) or an Si harness if you are not interested in mine.