I should be getting some new wheels soon so if you want to borrow some 18's to see what its like I'll let you know when I have a spare set. What about 19's? Bigger wheels on the Impreza do bugger the handling up unless you get the geometry set up properly afterwards and there is more unsprung weight etc, but thats never bothered me and mine feels ok on 18's. Not sure if coilovers will help as they are much shorter then standard shocks anyway so I'm not sure you will be able to raise the ride height over standard as even at there max coilovers could be 10-20mm lower if that makes sense?? I'm not sure on this mind, but even so your car would start to look strange if you raised it.
Heres a pic of mine side by side, the standard set up does compress alot more when on, but the D2's are still much shorter:
