Yep, I asked my local shop on Saturday if I would need to swap back to standard cans.
Response was the same - MoT rules have changed so as long as the 'not for road use' marking is not visible, or they are excessively loud, will be fine.
The guy said to me mine would be fine (carbon cans on the ST4S which are pretty loud imo, Ti cans on the 998 which are also pretty loud
)
Btw, he also told me if there just happens to be a sticker over where it should say 'not for road use', they cannot (or should not) remove it