If you get a blowout on the rear, take off the blown one, take off a good one from the front, and use that on the back. You essentially always want to swap up so you're always running two good tyres/rims with a similair rolling diameter at the rear, and use the space saver/normal wheel combo at the front.

From memory most Subaru rear Diffs are VLSD, and you'll tend to consequently do more damage to the centre diff. The front diff is open (generally!), so there'll be less strain on the Centre Diff if you've got offset rolling diameters on the front. If the transmission gets clunky and starts banging during tight turns; that's generally an indication that your Centre diff is on the way out, which is generally the most sensitive to offset tyres.

Experts, please correct me if i'm wrong - but that's my basic understanding.

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03 STi @ 280hp/1.3bar