The general rule of thumb for wear and tear for trackdays is 10x- 100 miles on track will cause the same wear and tear as 1000 miles on the road.
If you run on slicks, go for about 30 to 40x; you will destroy gearboxes and diffs, bushes etc... and the limits will be raised so much that if you have a surplus of speed over skill it's going to hurt real bad.
A properly set-up Impreza on slicks is damn fast though; I've lapped Oulton with Gallardo and GT3 racers in a (heavily) track mod'd STi- but we did destroy two diffs and a gearbox.
It all depends on how you drive, and how your car's set up. If you have proper diffs and gearset you'll be fine- but they'll destroy a road car if you're a quick driver.
Another word of warning.... take your time warming them. Driving on slicks is very different to driving on road tyres.