The absolute "best" Socket FM2+ CPUs you can use on that board are the A10-7890K (4.1GHz base/4.3GHz Turbo) or the Athlon X4 880K (4.0GHz base/4.2Ghz Turbo).
As to whether they'll provide enough of a boost...I don't know if it's going to be that noticeable. The base clocks are maybe 20% higher or so, but the Turbo speeds are only 10-15% faster than your current chip. Plus, Tom's Hardware has them on the same tier as your current CPU (http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html).
The primary benefit, however, from either the A10-7890K or the Athlon X4 880K is that "K". Unlike your A8-7600, it means they're "unlocked", so you could always try to OC them for better performance. You would need an aftermarket cooler, of course. If you decide to go that route, I would recommend the Athlon X4 chip; since you already have a dedicated GPU, you don't need the "full" APU with integrated graphics. That should help save you a bit of cash.