I drive ~50mi/day round trip, when I went through the F/I guy, he actually mentioned you can call BMW Financial and buy more miles (per mile is less than what you're quoting) as long as it's at least 6 months before the maturity date.
I'd give them a call and ask them. The reason I went this route is because I'll only pay for what I really actually use, rather than just buying 15/18k miles. They decrease the depreciation at the start. So it'd be a waste if I end up not using them.
2016 750i xDrive M-Sport (Current)
2015 650i Coupe
2013 750i M-Sport