I've used them, and lent them out with reasonable results. They do quieten the cans down and whilst they would probably get you through a track day, the MOT will be a problem as your Termis will still be missing the all important "E mark" or "BS" stamping.
Got some for the ring last year, just in case. Yes marshalls were doing their job. Bike failed their db test. !00 db at 1 meter. Ended up wrapping loads of wire wool around baffle insert. Finally got it down to 98db. so the weekend could continue. Vowed to use original pipes next time. Mot is really at the testers discretion, as they are not using db meters.
Silencers fitted as original equipment (i.e. are marked with manufacturers symbol) may not have BSAU or European approval markings, but are acceptable.
Not sure to be honest. One of the blokes I use won't pass 'em if they haven't got the E, BS or manufacturers markings on. If there's nothing on there or "not for road use" then he'll fail them.