If I replace the current catted mid section with a catless one, any views on whether or not the the car is still likely to get through an MOT (on the strength of the main Cat)?

Some do, Some don't, trial and error I'm afraid!

Mine didnt...

Prodrive lose the cat as part of the ppp, so you should in theory, have no problem.

mine passed with one large cat (non subaru) in the centre section.
:)

mine went through no problem

I decated my downpipe n got through fine :D

just make sure your car is warm ;)

my 93wrx just passes without a centre cat. It seems to be hit and miss but if tested at the lower limit of the required rev range it goes through (ie 2500 rather than 3500 if the test says 2.5k-3.5k)
It depends on the tester I suppose. The ones I used in the past all said "we'll get it through, don't worry" :)