Does EE/GS% reaches around 100%? Then it's cpu limitation, using some speedhacks can potentially help, at least using recommended ones should be fine without any sideeffects, or trying with sliders if you already had recommended on, through those speedhacks can and usually break things, and if you had them on from the start, you could decrese or turn them off, as they potentially can actually slow the games in more demanding moments and just fake fps.
If none of those % reaches 100% then it can be your GPU limitation, and you could decrease internal res in GSdx option or try to enable/disable "allow 8-bit textures" as that option can potentially speedup or slowdown the game alot at times, it's different for all games(and sometimes even works differently in same game at different scenes). Your GPU through should be good enough soo unless you did set your internal res too high, you shouldn't be limited with GPU.
Edit: also if your CPU limitation is GS thread(high GS%), you can try svn builds from here
, as one of the mayor changes - gif transfer rewrite helped with it's speed alot. And if it's EE thread, you could also try those builds, for MTVU speedhack, which divides VU1 thread from EE one, potentially speeding up lots of previously EE thread limited games especially on cpu's with 3+ cores.