No experiance in this scenario, but from previous I would always go with the expert recommendations here. If they say it'll be to loud and sound rubbish, it will be waaaay too loud and sound like a tractor mate
Yes, it probably is best to go with their advice, it's just hard to know for sure if the one chap you're talking to does know what he's talking about and there wasn't some ulterior motive, which is why I'd hoped for someone's first hand experience.
Milltek themselves test both versions, with their sports CAT, in their below video. Again, only so much you can tell from a video, but the race back doesn't ridiculous in comparison to the OPF back
https://youtu.be/TAPiMFbSLnY?si=vgQJEnNtj9St3lr3