i have a piper backbox with a decat on 8v. little noise on tickover but nice and loud/raspy with your foot down, can be driven quietly though
as for the MOT i wouldnt let that put you off a decat, takes 15 minutes once a year its hardly a chore lol
yeah the mot you know when its coming, but its the random VOSA checks going on that put me off it.. in the last month three people I know been pulled over randomly for it, one got a prohibition because he had a breather filter rather than a closed system.. had to replace it and go down to a garage to get re-tested and bring the new test pass & the VOSA paperwork to the police station to get his prohibition lifted!
its entirely your own preference, can't hate on anyone who wants to do it, but just me I can't be bothered risking it and having to re do it every year and then if i ever get VOSA'd, having to swap it, get it tested at an mot station, and down to the coppers, all hoping its done in a single day so i can resume driving. just not worth the extra few horsies to me haha