The problem is if I may be so blunt.......
the car has been lowered by the bodykit and not by a suspension drop.......hence why the car is looking so tall. 18" wheels will not make much difference to filling the arch spaces.
Whats needed now is a compromise......lower the car......and see if the side sill extensions will foul anything........if they do put the old MGR ones back on and drop the car some more to close the gap on the arches.
Just my opinion of course but I do like the kit.....looks very good.
Jim