The processor can not be changed out. If you're looking for more speed, I would suggest installing more RAM and a solid state drive. As far as technical limitations are concerned, the motherboard can handle up to 16GB of 1333Mhz DDR3 SODIMM, running at 1.35v. The brand doesn't really matter, as they all do the same thing.
Any 2.5" solid state drive will fit, and the prices have come down considerably. There are a lot of 480GB drives available around $130 +/- on Amazon. You'd notice a huge difference with just the solid state drive.