Hi Paul,
Car lovers always prefer aftermarket performance/beautiful wheels for there cars...............Not many of them knows about physics of car wheels as most of them are non technical..........Unnecessary modification of tyres/rims (to big or small) will effect safety, this may end up in to Break fail, balancing, Hard steering, tire burst and many more
I would suggest you to go to any Tyre calculator web site put ur company fitted rim tyre/rim info and u r new after market tyre/rim info. u will get data difference in diameters. as per standards recommend ±2.5% the MAXIMUM difference in diameters.*********************Please make calculations and proceed with your purchases.