after some digging - it does indeed look like they are different and the lips do fit flush with the bumper - there is also a trim line that follows within the moulding between the arch and the bumper - the examples I have seen also look like the runners / side skirts are different also.
got this info from this link...
http://www.evo.co.uk/carreviews/evocarreviews/50525/subaru_impreza.html
Over and above the established Spec C chassis, the 2005 chassis has a 15mm wider rear track and wider rims (up from 7.5 to 8.0in) with wider-spaced mounting bolts. This increase in what is technically known as Pitch Circle Diameter from 100 to 114.3mm comes as a result of fitting larger, stronger roller bearings for better wheel control. Also, 114.3mm is a more commonly used PCD that allows a wider choice of wheel for those rallying the WRX in Group N.
some wide track wheels
http://www.ebay.co.uk/itm/2005-onward-Subaru-Impreza-Speedline-Corse-17x8-Alloys-Wide-Track-5x114-in-Gold-/171336310049?pt=UK_CarParts_Acc_Wheels_tyres_Rims_Car_Wheels_ET&fits=Car+Make%3ASubaru|Cars+Type%3A2.0+WRX+STi|Model%3AImpreza&hash=item27e4709d21