Just buy the right sizes...
You have invested some serious amount of money into a nice looking and lovely to drive sports car. I spent over £30 on a night out the over night over the bar, probably the difference in cost for one of the four tyres - just skip a couple of nights out per year and you are almost at the point of recovering the cost. These things are going to be on your car for a fair while, why buy something that doesn't look right (and they dont on 18s) and could at some point cause other problems if you get it wrong as chris has pointed out.
You can play around with upgrades, uprevs, colours, looks, badges whatever, but just get the tyres right first - if anything those would be the things I would throw the money at and get right.