Good - more capacity for a "popular" attraction that commands long lines. Granted those long lines have as much to say about the lack of alternatives in the park it occupies as it does about the middling hourly capacity that the attraction offers.
Bad - after people ride TSMM with no 1/3 last wait... then what? The same issues plague the park that it currently faces. Not enough things for guests to do. After a slightly shorter wait, guest will be left with the daunting task of deciding what and where to go next. Until the real expansion comes in the form of more "things" not quicker moving "things", the net result will be that guests will likely do what the do now - leave unfulfilled and head to the MK.
While it's nice that a "popular" ride will be slightly less "popular", the priorities are wrong. Get more "things" in the form of rides and attractions online to make guests stay at DHS all day because they are so satisfied that no other draw on property is a better alternative after they enter the gates.
Talk all you want about Pixar Place and Star Wars land; but, until we know when the are coming and more importantly - what form they arrive in, we won't know if the problems facing the park are going to resolved or made worse.
If Pixar Place brings a bunch of "things" but those things don't offer good enough capacity or aren't high enough in quality to hold guest satisfaction, then the NEWNESS factor will simply bring more people into the park.
Star Wars land will be a big determining factor in the health of the park in the long term. If they only bring the land with a big E-ticket draw, then the addition of the land itself may cause more problems than it solves.
There needs to be balance.
I'm in the cautiously optimistic side of things with the 3rd track for TSMM. If it's arrival is followed by Pixar Place in the near future, it makes sense. If Pixar Place is still 3-5 years out, then the net result is treading water.
Soarin's new theater makes much more sense for the park its in. It will certainly help with the new draw in Frozen next year.