I guess it depends on how picky/thorough a claim assessor is in the event that there's a claim. A de-cat system will generally not pass an MOT, they could then argue that the vehicle was not in a roadworthy condition which could affect a potential payout.
That said, when my Lotus had a de-cat pipe on it I mentioned it to Admiral, my insurance company at the time, and they didn't seem bothered and didn't charge any more.
I eventually swapped to a sports cat as I got a bit fed up with the amount of noise after a few years. It was fairly obnoxious, not that I realised until I let someone else drive it and heard it! It also meant that I couldn't get on track days as it was too loud. Now I've changed to a sports cat it's much quieter but the sound remains much the same.
Is it really worth the concern and hassle of swapping it back on for MOT and will you (or your neighbours!) be able to tolerate the noise?
And finally, louder isn't necessarily better (heresy, I know). Just my 2p.