Just to add my two pennith...have you considered Maestro lattice alloys? They are 15" and the protrusion from the bodyside is less than that of an MGF alloy - partly through the different offset, and partly because they are only 5.5" wide (compared to 6" for the MGF alloys). I only add this as I originally had the 13" GTa lattice alloys on my K3, and it didn't look that bad. I progressed onto the later spec GTi 7 spoke alloys - which are currently on mine - but I recently found that getting the 185/55/13s for them is next to impossible. I have put a pair of Maestro alloys on and it looks ok.....
I intend to take a splash off the current unmolested bodywork, and make a set of arches, then graft them back onto the original body....