Have anyone done that ? In disc measures i can see that coupe disca are 40mm tall . And alfas are 45mm.
Now lets forget about holes for bolts.
Would caliper and carrier fit there and would work with discs?
I know hubs are same so in theory discs should fit without problem.

or there is any better/price efficient rear brake upgrades ?

i already read in this forum a cheap upgrade with latest audi/vw
rear calipers (same attack) and vented discs

Surely the Alfa is 5 x 98 PCD, against the Coupe's 4 x 98, so the discs won't fit

Have Alfa 5x98 conversion front and rear(entire hubs) and for me was pointless to fit bigger rear discs.
Coupe rear hubs and 156's not same, but i'm 80% sure caliper bolt holes distance is same.....

It would be interesting to know what you're trying to achieve. In all my time modding Coupes, I've never really felt the need to make the rear brakes work any harder - the Coupe is 67% front weight biased when its standing still, so when standing on the brakes, I wouldn't be surprised to find its in the region of 75% - 80% - the only job the rears need to do is to stop the rear end overtaking the front...

could you not use alfa 145 front discs on the rear with the audi or alfa set-up? the 145's are 4x98, 257mm dia and 20mm thick, same as the audi rear discs