I just had a long conversation with my local dealership on the same subject.
They just had a meeting with HD on this. One of the head guys that helped design the M8 said you do not need to tune the bike with just mufflers.
If you add a intake, or headpipe you will need to tune the bike.
I'm installing the S&S MK 45's next week, and I will not tune the bike till I add the intake and headpipe, after my first service.
My dealership said I would not have any issues with just adding the mufflers.
2015 Road Glide Special,White & Black(blacked out)V&H Power duals,4inch Rinehart's, Super Tuner, RSD Slam AC,CVO fender,CVO bags,RSD Clarity Covers, RSD Black Ops Slam 21/17 Wheels,Stage 3,585 Cams, Jim's V twin Headwork 106HP 118TQ
2018 Road Glide Special, Factory 47 bars, S&S MK45's, Bunch of CVO parts