Haven't specifically heard anything relating to AWD Subi's, but if you think about logically:
For example, a car has brand A on the front, and B on the rear (which is the most likely situation.)
If brand A offers a different level of grip to brand B, it 'could' 'theoretically' induce understeer / oversteer issues when cornering, 'potentially' greater braking distances (although the efficiency of the brake is more of a contributing factor), and lastly acceleration may suffer due to poorer grip. Having differing level of wear has the same effect!
If you watch any form of car racing, especially big rear drivers, they often say that differing tyre wear has a big effect on the handling.
Lastly, just about all dealers suggest rotating your wheels/tyres at every service for this reason, unless you have directional and/or staggered wheels/tyres... Oh, and road noise may differ due to compound differences!
When you're talking about tyres, they are the only thing b/w you staying on the road and sliding off. If you're considering only buying two new tyres then don't - buy all four... No amount $$ can suffice for safety.
As for warranty - wouldn't have a clue!
Hope this helps.
Cheers
