Well on the F30 other lines (non M-Sport) BMW was running a 5.9% APR deal.
As far as I'm aware, they weren't willing to this on the M-Sport (although I think Rjberry managed to get it) and I think you're looking at closer to 8 or 9%APR as I understand it.
As for discounts, I think you might struggle to get 10% at this stage (pre M-sport launch). However, am sure you'd definitely get that if you were willing to wait. The early adopters always pay the most!
I think the best thing you can do is sit down with a dealer and discuss your options.
Too many variables to list really - do you want PCP or HP?
Since you're looking to finance anyway, it would be more effective to look at how much you'll pay for the term - I wouldn't worry about the "raw" discount they give you - since it might be skewed by what you pay on interest.