Sorry to be a boring old fart but I have wasted countless years fiddling about modifying cars and so am these days like a reformed ex-smoker in my attitudes to a lot of things car related.
Unplugging the Soundaktor is free, Carista isn’t
(but these devices are really useful for monitoring fault codes and resetting things so a good investment either way)
I’ve no idea what an R600 costs but just by looking at who markets it I know it’s not going to be cheap. On a standard car these things achieve nothing, or at least nothing you’d notice. On a stage 2 car maybe it’ll do something.
The CSS exhaust does get my vote however.
I had a 2013 GTI PP and although it drove and handled really well it didn’t sound anything special.
A CSS exhaust does give the car a decent soundtrack, it’s properly made and even as far as your insurance is concerned it could be argued is a standard part “yeah, the car came with it on, it has a VW part number on it and so I assumed it was standard fit innit”
Folding the back seats down does enhance the sound a bit too I discovered when I had my Clubsport Ed40 transporting IKEA crap home. So much so I kept the seats folded afterwards
(I expect just removing the parcel shelf will do exactly the same thing)