The 225/45 tyre was never offered by Alfa for use on the 156 / 147 except on the GTA (which has different offset and wider wheels)
It may be the correct rolling radius but not all 225/45 tyres will fit without rubbing on the front suspension, rubbing the front inside arch plastic on full lock or catching the rear arch when fully loaded - which is probably why Alfa used the 215/45 tyre as a safer option that would not result in complaints from customers
I run 225's, and the car is lower on Koni Sport Kit, around 25mm lower than standard veloce and 40mm lower than base model.
There is no rubbing at all, Clearence at the front is minimal (against upper wishbones), but that said I may give 235's a try just to see if they clear (more for track days than road if they do)
215 is the ideal tyre size for a 7J rim
225 is ideal on a 7.5J rim rim.
I don't know, but I would guess Alfa just follow the ideal size for a 7J rim? Being a 215.
Anyway, all that said, no clearence issues even on a lowered car.
(The turning circle on a 156 is that bad, I doubt 255's if they would fit would rub!)