On Q4 front and rear diffs are std open diffs and center is torsen.
Torsen gives same action as std diff but has a torque biasing ratio. So as you've said the front wheels can spin faster than the rears or vise versa but only to a point where the torque bias ratio is reached.
The rear tyres you ask about fitting have 0.3mph increase/30mph over the fronts, so as you increase speed the rear mph speed difference to the fronts increases too. Although you've selected tyres that have a very small speed difference, Alfa didn't know your going to understand enough to do this and it could be a far higher front/rear mph number that would increase wear/heat in the whole drive train. This is why the hand book states like for like size and brand of tyres.
You can see in this case your inducing "push" "understeer" into the chassis from higher rear wheel speed (albeit tiny) this will result in instability and only get worse the faster you go
The wheel speed difference will also bring in your traction control earlier and vdc too. I can imagine the various ecu's having quite a party just trying to stabilize wheel speed depending on how fine traction/abs/vdc wheel speed setup is in the software.
So now you are armed with what could go wrong, its up to you if you want to try it.
The torsen Diff is a "Double C". The "C" also standing for type 3. The Front drive shaft differential is within the outer Torsen diff. One half of the outer Torsen drives the inner, whilst the other drives the PTU - Power Transfer Unit, which has a Crown Wheel and Pinion to change the drive by 90 dergees to feed the Prop Shaft. Both the front inner diff and the rear diff are open. It is the ABS system that detects the same axle slippage and the VDC then applies "Pinch" to what ever wheel is slipping on the same axle, to ensure drive continues to go to the wheel with the most grip.
The outer diff of the Torsen is essentially a Limited slip diff. As the Torque, front to back changes, the Slip Rings/Torque washes on the shafts bearing the Planet Wheels, come under pressure from the Planet Wheels, which have helical gearing. Naturally they want to move away from the force that is driving them, causing them to exert pressure upon the Slip Rings/Torque Washers. This pressure arrests the tendency for the Planet Wheels to move around their own axis on the shafts to which they are mounted. This function creates a continuous shift in front - back, back - front torque bias. Up to a pre-determined designed level. At that point, the Torque Washers "Crack-Off" and the Torsen becomes an "OPEN" Diff. Once the torque level falls below the design level, the Torsen "Closes" again. It is so good, that it is probably the finest four wheel drive system currently available as it is for all intents and purposes, "Pro-Active" and not as is the case with all other systems, "Re - Active". But it does rely upon the fundamental principal that all wheel and tyre sizes and tyre pressures are maintained correctly. One cannot improve upon this system, no matter how bored we get, or how clever we may think we are. Quite simply, It is "SUPERB".