Unfortunately, here's what I *think* happened. I have an email out to BMW to comment.
2012 MY 328's have 10:1 N20's tuned low
2013 MY 320's have 11:1 N20's boosted low
~2013 MY 328's have 11:1 N20's boosted high (see my post about piston being discontinued)
This would mean that BMW would put the same engine with the same CR in both 320 and 328. Makes sense for simplicity sake. The bad thing (for 328 owners) is they will already be tuned high (most likely more boost from factory). So, from BMW's perspective 328 owners can no longer piggy back to ~335i power, but 320 owners can piggy back to 328 power. This is oversimplified, but hopefully you get the idea.
If a "legacy" 328 needs a new piston all 4 will be replaced with the 11:1 CR ones and a software update. This whole setup means that BMW has to only maintain 1 set of pistons and two tunes. Not 2 sets of pistons and two tunes. One way to confirm without digging into the engine is to check new vs old 328 boost pressures, which is probably where they would make the adjustment (like a piggyback).
This is all speculation, but might be worth looking into deeper.
EDIT: This jives with namelessman's post. Power increase from boost would primarily be seen in the HP range.
Last edited by Ronin951; 01-18-2013 at 12:04 PM.