1. Remus race produces a bit more sound compared to the Sport version. Also race is slightly lighter in terms of weight (has the two tube muffler design vs the box that the sport does). There's been a few people who have had both and posted reviews on both, so I would look for those threads.
2) No tune needed. And you will remove resonator and secondary cats with the AA mid pipe. The mid pipe acts kind of like a giant resonator itself and with the primary cats intact, you should still be able to pass emissions (I have not checked yet personally, we're exempt for two years here if we have a new car).
3) No CEL.
4) Catless DP from reviews sounds like you will have about a 30% increase in sound from stock pipes.. However there are reviews saying catless will ruin the sound and induce rasp back into your AA mid and Remus rear set up. Personally, I think the car is plenty loud with OEM DP + AA mid + Remus rear already compared to the stock set up and I would recommend going that route first and maybe doing catless afterwards instead of all at once.
5) Efficient mode in AA mid and Remus race is like stock. Just as quiet, pretty incognito, no burbles, no real increase in volume.. Sport+, the exhaust turns into beast mode. Burbles are more pronounced.. you will hear all the burbles now... The over run is pretty long. Cold starts are really loud now. The BRAAP noise is LOUD... in tunnels it will hurt your ears if your windows are even cracked
. 5k+ RPM, the exhaust will wail/scream. That "lawn mower" sound is minimized and the overall exhaust tone is deeper and louder. Honestly it's the Jekyll and Hyde of exhausts when you compare efficient and sport+. Also butt dyno felt a small increase in power. No drone as well. Overall, really happy with the exhaust combo.