Hi Robin.
As ever its all about balance, recently had a go in an S3 that had R8 rear brakes on the front and S3 rears on the back, the reason for the comparison is that the R8 brakes have the same 4.95 cm/sqd piston area as your AP's and to be honest it just felt great
We've all danced this waltz before about whether or not its a worthwhile upgrade and I would say yes it is, when fitted the car definetely feels more stable and therefore inspiring more confidence. The Danza's of this world need the weight transfer to the front as it improves traction especially when considering that ultimately when passing or racing you will probably end up braking and cornering at some point which is something your average clubman track day slagger is told to avoid.
My point is the tyre contact patch is actually wider left to right than it is front to back so if you're really going for it you need the weight transfer that small rear brakes give you, invariably we want to go fast but not grow grey hairs at the same time due worry about stuffing it.....
I would say chuck em on and see, its an experiment, if it doesnt work out you've spent two hours of labour in getting them on.
If you were planning on taking advantage of the reduced stability of the smaller rears then Im guessing that you would need to change ARBs and tyre pressures etc to get that "loose" feeling.
Its all about driving style - If you try them on and dont like it I'll have em back!!