I think you guys need to pump your brakes for a moment and wait til you have a production vehicle.
First off... I 100% agree that the ride height on the pre-production vehicles at Leipzig and even at the different venues like the Detroit show have the car sitting a little higher than I would like. So I understand your line of thinking. However... the group of us at Leipzig were cautioned that the M division was still fiine tuning the suspension setup, so it's a fair likelihood the production cars may be different. Looking at the 335, and other current models, even the 5 series and the current 135... I don't think these cars suffer from looking to tall stock. As a matter of fact, I look at the current 335 and say.. wow.. that car looks pretty low to start off with! So let's not get too overly concerned with the pre-production cars...
Next.... let's think about the process of putting together the pre-production cars. Doesn't it seem likely that they simply used springs from the M3 parts bin to start off with? And if so, then these are the springs that we feel have the car to high?!
Why is the car sitting too high currently? Could it be that the springs currently are too stiff because they are designed to support the weight of an E92 M3 that is 300-400 lbs heavier?
If you are considering aftermarket M3 springs, aren't the aftermarket springs going to be STIFFER than stock? I don't know a lot of companies that make aftermarket setups SOFTER than stock.. So.. if you get some aftermarket M3 springs, sure they may be shorter, but shorter and too stiff already, on a car with less weight than what the springs are designed for seems like it would make for a very harsh ride.... Not to mention the OE shocks are not designed for such stiffer springs... that just seems like a recipe for disaster.
It seems much more prudent to wait for the production car first, and then yes, if you want to lower it, maybe wait for an aftermarket company to test the OEM spring rates.... If you want to be the first person on the block to lower your car, maybe sending in your springs to a tuner like AST would be a good place to start, so that they can determine what the factory spring rates/heights are, and then proceed from there.