Focal Sib 5.1.2 "up-firers" vs. Kef T-series 5.2.2?

[Scarriere]

Your Focals are nice speakers.
I'd avoid changing your front stage to those T's. In my opinion, it'd be a step back.
Putting the T's above you for Atmos is a decent idea. Are the front upfirers defeatable? Do the front speakers have two inputs and you can take the Atmos input out and put it on the T's for above you?

Having your surrounds behind you rather than beside you isn't that big of a deal. Whatever room calibration that you've used should read and compensate for having the S/L close to you. If not, just turn that level down a hair, but not too much.

For your sub choice, it seems size is a big concern. For that, I'd avoid the sub you mentioned and go strictly with a SVS PB-1000 or two. There's a very short list of subwoofers that are smaller and still perform well. That SVS is likely at the top or very near near to it. The price is attractive as well.
Myself, if space were a major concern, I wouldn't consider any other.

Your room layout would be helpful.
I think you're getting on the right track.