I'll answer that I can
1. The catbacks should swap over just fine, I have an IRS catback behind my parents house, that is just sitting, as I was planning an IRS style exhaust on my 03 SRA but never got around to it
2. The lengths are a bit different, look up cobra 4R70W conversions and you should be able to find out the length needed. The only difference in length between going 4R70W(auto) to T-56 vs. GT 5-speed to T-56 not accounted for here, is the yoke, but that swaps right over and does not need to be accounted for as far as I know. Basically you need to know the difference in length
3. Not really sure here, people put cobra rears on the SRA all the time, so no idea why they wouldn't swap. More than likely it is an issue of the person with the IRS not wanting to lose the nicer rears.
4. You will need new lowering springs that match the IRS in the rear, and also most likely will need different shocks, IIRC the mounts are bigger for IRS, and the bolt hole is larger due to this. You also will want to look into a FTBR bushing kit for the IRS.
Hope this helped!
2003 Redfire & Black V6. Full suspension, 8.8 w/ 4.10s, tuned. 4.3L stroker + turbo capable of 550 hp soon
1992 5.0 Auto Hatchback...needs wiring, interior, ...basically everything
RIP: 2015 SHO PP, 12.87 @107 1/4 mile