I am very interested in getting spacers for my stock rims on the 135i, but I swapped the stock runflats for Yokohama S-drives.
Same tire specs as stock at 215-40-18 F and 245-35-18 R, but they do appear to be wider due to squarer shoulders.
Tire Rack specs the section widths and tread widths to be almost identical as the stock Bridgestone RFT's, but I wonder whether the Yoko's will run with your suggested 18mm front and 10mm rear spacers.
The poster with the Dunlop Direzza's indicated he rubbed, but his tires were 225F and 255R.....which are about 1/4 inch or 5-6mm wider.
Any experience from customers with the Yokohama's on this setup?