FM2 is designed as a socket with video out capability to take APUs - you won't find much other than APUs to fit your board..
You need a Socket AM3+ board to take the really high performance "FX" CPUs.
The best you may be able to fit in your existing board (if the BIOS supports it) is an A10-7850k
http://www.ebuyer.com/613105-amd-a10-7850k-black-edition-3-7ghz-socket-fm2-apu-4mb-cache-retail-ad785kxbjabox
That's actually quite a reasonable combination with the R9-270 as the CPU itself is around 2.5x faster than your A4 and also that specific pair will work in "crossfire" mode, with the GPU in the A10 boosting the performance of the external GPU.
(The GPUs must "match up" & I believe only a couple of the A10 versions pair with the 270).
You need a 450 - 500W PSU for that combination.
If you are going to try that, see if you need a BIOS update before swapping CPUs, so you don't fit the A10 then find the board cannot boot as it's not recognised.
There are some performance figures for the A10 and that paired with the r9-270 on this page:
http://wccftech.com/amd-kaveri-apu-a107850k-gaming-general-performance-unveiled-mantle-45-faster-directx/