gscotts - not everyone pushes the extremes of offsets to use a tire of that size. A lot of 20" upgrades use the standard 255/35/20 and 275/30/20 due to its easy availability and cost. The sizes you are mentioning requires offsets that most aftermarket wheels don't make unless custom ordered. A 25 profile rear tire is very thin and its not for everyone.
If you can run those sizes without having to roll the fenders or add more negative camber to make them tuck then go for it Please show pics when you are done.
The 21's looks great too but I think the odd size makes them not as popular to most.
The stock wheels for the M5 is quite liked by many so they stick with that. The stock setup with spacers and springs looks aggressive and provides the performance and handling that many want to achieve.
Thanks, I appreciate the replies. I have dinan stage one suspension. Was told by hre it would be no problem. Just curious as to why more people dont have the sizes i mentioned. I had my stock wheels refinished in gunmetal and have snows on them. (set is dedicated to winter duty), I want to maximize width for the summer setup.