I doubt the extra track will have much effect on wait times beyond the first couple of months. Amount of guests experiencing the ride, or times they go on it? Probably, but not the times themselves.
Firstly, they may just fill the extra track with Fastpass+ slots, and because Disney makes it compulsory to choose 3 attractions, they'll easily sell out, and go to people who woudn't have bothered before or think 'why not, if they're giving me a free Fastpass, maybe I will go on it again'.
And secondly - most importantly, I think - not adding attractions does nothing to solve the issue of repeat riders - it's a very repeatable attraction, and I would argue one of the key problems is the line isn't just made up of one and done guests, but many people in that 60 minute line are trying to beat their score, or simply are going because there's not much else for their family.
Those are the crucial guests you don't get in California, which is a key reason lines are shorter there. With so much to do in Disneyland, nobody would dream of riding TSMM twice in one day - there's no time - but by not building extra attractions, those repeat guests aren't going anywhere in DHS and will simply try again, maybe a third time, if lines seem to temporarily drop, soon filling things up.
Finally, you have a large percentage of park guests currently not bothering because of the lines. Those are the ones who will fill up any leftover slots, and before long you'll be back to where you were.
Hope I'm wrong, of course.