Would a modified Coupe in the region of 300-350BHP with a 100 Cell sports Cat be expected to pass the MOT emmissions test

Maybe. No garantees. You'll see some people jump in here that say "yeah but mine passes fine". Well it might pass now, but over the years the emissions will get worse and a 200 cell will see you pass MOT a couple of years longer.

I know of 3 people that didn't pass MOT with a 100 cell, and I know of one that passed emissions without a cat.

the main thing is the cat is hot,if you have a good relationship with the garage get them to test the emissions first while the car is hot not after they have let it cool down on the ramp while they check everything else.
otherwise let them do everything then go for a drive to get it hot before they do the test.