From what we've been told there doesn't seem to be a hardcap on experience or levels, just a point in which you will run out of things to earn experience from (in other words, there is a finite amount of XP available). Of course, the problem there is that one player may consume 90% of it before reaching the end of the story while another player only gathers 60% by focusing solely on the main plot, and the designers may have designed the final encounters around the assumption that players will have accumulated 75% of the XP available.
But, yes, if there were a hardcap it would be easier to predict/control how far everyone has advanced by the end.
Gotcha, I was under the impression it would be capped (not quite sure how I came to that conclusion though!). If that is the case then yes, it would be very possible to be overleveled for the final encounter, or any encounter for that matter. I'm with you in that I sincerely doubt the developers would design the main plot's final battle with the assumption that a player has completed every available quest and side quest. That could make that last fight a bit of a walk in the park.
I don't think there is much to be done about this though. There will be players on two ends of the spectrum (those who want to mostly advance the main plot line and those who want to do every little thing) and the final encounter probably needs to be balanced somewhere in the middle. Likely it will be balanced with the assumption that the player completes at least a fair amount of the side quests.
