How much of a boost should I receive here? I unlocked a core for my CPU making it from an Athlon x3 450 @3.2GHz to a Phenom II @2.7 x4 (I had to underclock it because the power draw was too much).
I usually get around 50-60 FPS on some games
You'll probably do worse, as, unless you use LLE on thread (which you're way too slow for), Dolphin can only use two cores. This means you won't get much of a speedup if any, and you've also underclocked it, so the tiny speedup is completely negated.
I agree with AnyOldName3. You should be seeing lower maximum frame rates because you underclocked. I just did some tests on a Phenom II x2 unlocked to an x4 then overclocked, results in the windwaker benchmark thread. All other things the same the extra cores just stabilized the frame rate, ie. increased the min fps. Overclocking increased max fps.
Based on my findings dolphin max fps is tied to CPU speed alone, at least on a phenom ii platform, core count helps maintain that max fps.
Disclaimer: My testing was nowhere near exhaustive and your results may vary.