I worked under the assumption that BMW dealerships mark up the cars by 8% to get the MSRP. So if you can get an 8% discount that would be great but I think highly unlikely as that dealer won't be in business for long. It really boils down to what you are willing to allow the dealer to make on the deal. Decide that number, stick to it and be prepared to walk out the door if they won't go for your offer. There could be more complex things going on with pricing but I don't think BMW offers many manufacturer to dealer discounts for M3s.
Dealers may be more motivated to sell cars on their lots as everyday they are making interest payment on those cars. However a newly ordered car costs them nothing.
Also if M3's are a hot item in your area, maybe you have to pay full price. Hope not.