Just made this great wheelie.. did you see it?!
Join Date: Sep 2018
Location: Canada, ON
Bike: 2018 R6
Hi, I did something similar with my 2018,
1) yes it will be crazy loud, I am using stock header with the cats removed along with M4 mid pipe/ yosh TRC slip on. When I have the baffle out it is too loud for street riding my ears ring.
2) you probably want a tune, the MJS headers are the same size as stock they just don’t have the cats. I ended up going with an auto tune for mine (flash tune) (you might be able to use a power commander tune from older R6, seems to be a bigger library of tunes for the older gens, then copy the fuel tables into your 2019 Map) however that being said I would recommend flash tune bike side harness you will use a lot more features down the road, like reducing engine braking and turning off your x-up valve light that your new exhaust will cause.
3) if you leave your AIS system installed it will pop a lot on decel. I ended up removing mine and installing graves block off plates so I could get a cleaner tune. (Once again flash tune will turn this light off)
Have you considered punching your cats out in your stock header to save the $$ vs buying the mjs header?
Let me know if you have any other questions.