How would you compare Cannon's Digic 5 + v/s Apple's 5x. I have searched the Internet for answers and all that I find is that the Digic 5 + is 3 times faster than the Digic 5 and 17 times faster than the Digic 4.
I don't have a quantitative answer to that, but most of the magic in Digic is in the custom image processing system. The ARM core appears to be rather ordinary ARM 946E-S reference implementation, which is several generations behind smartphone processors, and not remotely comparable in performance.
What stops the Canon's and the Nikon's of the world from opening up their systems and allowing me to write Apps for it...
Nothing except motivation. Clearly, they don't think they will make money on it at this point.
As it is most camera manufacturers buy the Image sensor from a third party, get someone else to build the processor and also the lens.
Canon designs their own processor (the ARM part is licensed, but the image processor is not), and their own sensors in some cases. I believe the same is true of several other manufacturers.
If they can outsource most of the camera would it not be in their interest to also make the system more open as the differences between camera manufacturers models is not very much.
For the moment, they are still trying to battle it out with more zoom, more megapixel, more software features etc.