It really depends and it's not something that is easy to explain but here goes. Any increase in speed is usually offset by the increase in timings to yield almost identical performance. But there is a slight edge you can gain, and I'll explain through an example.
Say you have memory running at 667MHz and 3-3-3-8-1T. You slowly increase the speed a few MHz at a time. At 730MHz, you have to increase the timings to 4-4-4-12-2T or the system won't post. You continue increasing the speed a few MHz at a time. At 980MHz, you have to increase the timings to 5-5-5-15-2T or it won't post. You continue increasing the speed a few MHz at a time. At 1120MHz, you have to loosen the timings to 6-6-6-18-2T. At 1160MHz you have to increase the timings to 7-7-7-24-2T. At 1210MHz it doesn't post no matter what timings you set.
Ok, if you're still with me, here's why this example is helpful. You cannot just set the memory to any speed you want. There are a few dividers you have to choose from, and these dividers place your speed somewhere in this range. Lets say you have one divider that puts your memory at 960MHz, and another divider puts your memory at 1040MHz. Well at 4-4-4-12-2T timings are stable for any memory speed below 980MHz. Thus, 960MHz is close to the fastest speed obtainable with these timings, and should give very nice performance. 5-5-5-15-2T is stable at anything below 1120MHz. Well 1040MHz isn't very close to this, so the performance is likely to be worse than the 960MHz at 4-4-4-12-2T. Does that make sense? Every set of timings has a range of speeds for which it's best to run those timings. You want to choose a speed/timings combo that puts your speed near the max speed for that set of timings. Sometimes higher MHz wins, sometimes lower timings wins.
Another thing worth pointing out (and keep in mind these are made up numbers but the trend usually exists) is that once the speed gets too high, loosening timings has less and less of an effect on stability until it just flat out doesn't help with stability. So it's usually best to keep the memory's speed somewhat close to it's specs, or the large timings increase is too damaging to performance.
But as others have pointed out, benchmark benchmark benchmark! It's the only way to know for sure.