I have a M&P 9, standard 4.25" barrel.  For a while I was using it for everything (carry, hunting, target).  I am going to pick up a M&P 9c, and was planning on bringing that for hunting and use it for carry as well, so was thinking about getting the M&P9L or Pro for target and possible future competition.  The problem is, I am a CA resident, and neither model is available.  Does anybody know if you can buy the slide and barrel and throw it on the regular frame?  Thoughts?  

Is the 9L/Pro that much more accurate than the standard 9?

You will have no problem putting a 5 inch upper on a FS frame. The frame is the same. The difference is in the slide. I would say it is more accurate and is well worth the switch if you can find a slide. Dont know if you have the ability to but if you can i would get a 40 pro/L slide if possible with the ability to switch to a 9 later or also for uspsa you can shoot major or minor.

The recoil impulse is a little different on the 5" versions vs the 4.25".