Well, I am a proponent of superthrive, I think it's great stuff... but I think it can be overused as well.
Superthrive is basically growth hormone and VB12, and it doesn't effect visible foliage like many would think, it encourages the plant to build stronger roots, and that is it, this helps your plant deal with stress by giving it a healthy root s system to draw upon.
I can't back up what i am saying here with truth, but I can tell you I haev a few passion vines that are around the same age... and this may not sound like an endorsement, but in a way it is... 2 passion vines, nearly identical (same species) planted in identical pots within 10 ft of one another... One I used super thrive on, the other I didn't. (I stressed out the one really bad the year before.) anyhow... long story short... the one that has been superthrived looks almost identical to the other one... with one major exception... when I water them, the one that has been superthrived goes through it's water twice as fast, and in a pot, and the texas heat, droops faster... interesting... why?
Stick your finger in it's pot and you will find nothing but roots... and I mean that, it is the most root bound plant I have ever seen and I can't afford to re-pot it... were talking... 100 dollar pots here to fit this bad boy... his root system, in my opinion, is OVER developed.
THe other passion vine seems to be more evenly adapted, and is par for the course with the other few that I have, it's getting root bound, but isn't quite there yet... certainly not to the degree that the other is.
Take it for what it's worth... I think you get better roots out of it, but I can't prove it... and prooving it, is what remains superthrives perpetual problem.