Tino, Matt's comments re toe seem right if the car is squirming under brakes...rear toe will also make the car nervous through corners. Basically what one tries to do with a Subaru is dial out understeer using various means...your car has maybe tipped too far the other way. You should be running zero rear toe, then about 2 deg neg camber rear and 3.5 to 4 front....KISS principle. I would strongly suggest a Wheels World alignment.
The tyres will easily come on song within 3 laps of the track...if they are DH's old ones they are probably R's, so should be spot on. The hot temp of 36 sounds about right...ie starting at about 32/30 F/R cold.
I would also recommend a couple of sessions at RAC...firstly an hour with John Fowler to fine tune the handling then a one-on-one with someone like Kim Screaigh who will help you with technique and lines. It is reasonably easy to tune your driving at RAC if you really religiously stick to the witches hats lines...like I always tell peolpe in the V8's, just go from one to the next like you're joining the dots on a drawing.
The other bit of advice is to get the handling right, then LEAVE IT ALONE (ie no add-ons, changes etc) and just drive it.

Good luck!

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