In time, I would not be surprised to see BMW replace what we currently think of as the top trim level of each current model in the range with an M Performance model. In other words, sometime in the future, perhaps the 335i will go away and there will just be a M335i (or M340i). Similarly, the 550i may go away, replaced by an M550i.
This would be similar to what Audi has done (at least in the US), with the latest generation S4, S5, S6, S7. I.e. the 6 cylinder A4/A5 is the S4/S5 (and yes, I know the coupe still has the V8 for now), and the 8 cylinder A6/A7 is the S6/S7. Of course, the S models offer other enhancements from the A models too (such as suspension, brakes), just as will BMW performance models vs. normal series models.