If your going with a custom exhaust and you want the most out of that LS/V-tec motor for natural Aspiration then you want 2.5 mandrel bent from the back of the header all the way out. As for the ecu situation trust me if you get an OBD0 to OBD1 conversion with a p72 ecu it will run the motor right and there will not be any issue's with v-tec cross over. Since you do not have the stock GSR intake manifold it will throw a check light for the fact that is doesnt see the secondary butterflies, but that's all it will say nothing more. Right now the ecu you have is not running your motor right, so even if the gsr throws a check light it's still running the motor correctly. Just get a P28, a chip kit and Ill custom burn you a ROM for your setup. No check light, and no issue's.
The PW0 should be turning your v-tec on at 56-5700 and redline 81-8200, if it's not then it's bad. Redline is lower cuz of the valve train.
The P72 will turn the secondaries on at 4400 then v-tec at 56-57 and red line at 8100. I know this for a fact since I have run about 4 gsr ecu's in my time. Redline was set this way cuz of the shit ass R/S ratio the gsr has.
The P30 will go v-tec at 56-5700 then redline at 8500, again I know this for a fact since I have ran several jdm and usdm. The valve train issue's were solved which is why the redline is higher, that coupled with the fact the B16 has a near perfect R/S ratio is why it will go to 9k with zero issue's. <----verified by me for 4 years.
I have also run several chipped P28's, one chipped P72 and one chipped P30. All wer ecustomed tuned by me or chips that I bought from others, and again none of the pre done chips worked right, the stock ecu's ran better than the pre done chips. However my custom tuned ecu's ran better than anything else I ever had.
As for the custom exhaust, Im going 2-1/4" piping because of my ticket. I believe the piping I have in there already is 2.5. It just doesnt fit the headers correctly, and there is no muffler. My cousin is going to hook me up with the conversion harness hopefully this weekend. Also, I believe he has either a type-r or gsr intake manifold. Ill try to get that from him next check. Ive decided that ima go with the GSR comp for sure. I just cant believe that all this stuff is actually going to happen soon, as opposed to me saying something and waiting 8months for it to happen. Also, I manage to sell the fender from my other car, so itll be a lil extra money in my wallet.