There is absolutely no reason why you cannot upgrade the rotors on the AE86 strut - stub axle assembly to 300mm or bigger.
The stub axle is quite a strong assembly and will be able to handle the added pressures of such a rotor set-up. That is not to say that there are not stronger stub axles out there, but why bother when the standard one is quite able to do the task.
If you want to you could always gusset the stub axle. Then there would be no issue at all.