Great timing for this question as I am going through the same dilemma!
I have been chasing Hp/torque ever since riding Harleys (since the shovel head days with that “monster” 74 cubic inches) and I now have a 2022 CVO Road Glide with 117”, and it’s plenty of power for me and is the first Harley that I’m not messing with the engine.
So here is my opinion:
1) From a couple of tuners that I’ve talked to, the stock headpipe is a great quality headpipe. Also cat in versus cat out, they have measured and there is not a significant amount of heat reduction…some yes but the M8’s are not that lean and do not run as hot as the twin cams. If you had a twin cam, changing headpipe in a touring Harley was a must! If you still “feel” that the headpipe is too hot, you can always add this:
https://deipowersports.com/products/heat-shield-liner-17-22-hd-touring-harley-davidson-flh-flt-flr-including-trikes-stock-headpipe-1/2) I talked to Fullsac recently re cores…2.0 versus 2.25 and the recommendation is to go with 2.25 with techmat…no loss in hp/torque going to 2.25” cores if staying stock with the stock headpipe. Harley will definitely sound better and more like a Harley, but if you are looking for really loud, this is not the right setup for you.
3) Do you need a tuner if changing just mufflers? Most will say no and not necessary, but I plan on adding Screaming Eagle Pro Street Tuner with the Smart Tune Pro. Overkill? Maybe, but I want those wide band sensors as I believe there are several benefits to them. A good Harley shop will have the necessary Harley kit to drill out the bungs for the narrow band sensors in your stock headpipe, tap and install the wide band sensors.
Don’t shoot the messenger, this is just my opinion and some will disagree with the above and that’s ok, but this is what I will be doing in the future…oh and everything under warranty (cue “screw the warranty”😀)!