The choices in a racing series are determined by the rules. Displacement, weight, suspension and engine mods are all set by the rules.
You can be sure that in such a competitive and prestigious series that BMW pushed it to the limit along with the other manufacturers. I doubt that they "left money on the table". You could surely get more power out of a normally aspirated 2.0 liter if that were the only limit but not with a production block, head, and possibly intake restriction to further equalize the cars. As noted in the article, there may be further weight adjustments of up to 80 pounds as the series progresses. Win and get added weight for the next race.