The horsepower wars will diminsh with time due to CAFE stds and physics. Increasing HP 20% as BMW did between the e46 and e90/92/93 will not yield the same gains as 20% on top of the e90/92/93 chassis. You ca't mess with the 2nd and 3rd laws of thermodynamics.
This all means that what we have today is pretty spectacular and BMW will be hard pressed to deliver significant gains on the next model comparable to what we saw with this model. The only way to seek parity in the gains is to (a) increase HP while ( b) simultaneously decreasing weight. SInce this is really hard to do at the price point that exists, I would anticipate just 10-15% overall gains with the next model and a lot of "content" to help bulk up the sales story.