Changing the angle of the trim tabs has a similar effect as prop angle. The best approach though is to set your prop angle first, then adjust your tabs mainly to suit varying water conditions.
If the water is calm, you can angle the tabs up a little so they are clear of the water and not causing any drag - so giving the fastest speed. In choppy water though, you are likely to want your boat to run with a bit more hull in the water. This gives more stability and often a faster overall speed as the boat is running more steadily and not "bouncing" across the chop. By angling the tabs down, you force the front of the boat to ride lower and so keep more of the hull in the water for greater stability - but some loss of speed.