Lower gearing (higher numerical ratio) in the diff means you will get less speed in each gear and hit redline at a lower mph in top gear. Raise the redline to 8600 and you will recapture a little of what is lost.
Go to www.diffsonline.com
and select the rpm/mph spreadsheet under the technical tab. Plug in the E90M3 gear ratios, at least as many of them as you are interested in since it shows only 6, and the differential and tire sizes. Vary them to experiment and you can see rpm at various mph with various gears and tire sizes.