If you’re planning on removing OPF then yes it will need to have a remap, if you don’t mess with the OPF then I’m sure you can upgrade the exhaust with no issues, although still not sure how much better the sound will be?
I went stage 1 6 months after buying the car new and best thing I could have done, added performance was great but the extra sound with pops/burbles was my main goal and makes it 10x better!
Understand reservations with warranty but I wanted to enjoy the car now and not wait 3 years, if you get a decent company to map it and look after the car then you shouldn’t have any issues hopefully.
Interested to hear your feedback from Miltek and any information on added sound as I’d welcome anything a bit deeper sounding, although spending £1,500 to only get minimal sound seems a bit much.