Thanks for trying motion-mmal on the Pi 2! I'm not surprised at lack of noticeable speed-up; however if you're seeing a lower frame rate, that's odd. Are you building motion-mmal yourself, or using the Dropbox binary?
To get a chance at faster motion, you'll need to build the motion-mmal binary itself for ARMv7 (the current binary is ARMv6 for the original Pi) - and you'll also need to get hold of (or build) ARMv7 binaries of all the libraries it depends on (e.g. libffmpeg) - some are statically linked to the motion binary, and I don't know if Raspbian on the Pi 2 has ARMv7 shared libraries as much is still used in common with Pi 1.
With the four cores, there's potential to parallelise some of the image processing steps - but that will require rewriting some parts. The ARMv7 chip in the Pi also has the NEON vector floating point unit, which could also help.
I have a Pi 2 now, I just need to find the time to get stuck back in to motion-mmal as there's now more interesting stuff to play with!