I've also got pirellis front and bridgestone rear with no handling or stability problems. I also raised this on the forum some months back - concerns and responses were based around any slight differences in circumference, tread patterns etc which might confuse and possibly damage Q4 electornics and diff? To add to this, the owners manual stipulates same make and brand all round and I raised this on this forum some months back.
As above, I've had no problems and I feel the only real risk is your insurers view should the worse happen and you become invovled in an accident. So the slight worry is would they pay out should the different tyres per axle come into question? Again, not come across any such story before so I personally don't worry about this.