Boats are designed to ride on an even keel at a specific bow up attitude, that's where they will plane most efficiently. Trim Tabs will allow you to achieve whatever attitude is called for in any given condition. Spray coming up off the port bow with wind on the beam, put the port bow down a little to knock it down. Ice melts, fish come aboard, refreshing beverages get consumed, the wind on the beam puts her on her ear, all can be corrected with trim tabs "on the fly."
As Doug mentioned, they will help you get on plane faster and smoother and stay on plane at slower speed. If you are looking to extend range on plane being able to stay on plane at a lower speed will do that and tabs will be paying for themselves.
Just like with airplanes, the faster you go the less "wing" you need. Our "slower" boats call for a little more than the common rule of thumb which is 1" if tab width for each ft of boat length. I have 42" x 12" on my 200HP 34 Mainship, need every inch to wallow up onto plane.
I recommend Bennett for too many reasons to list. The 42 x 12 is the largest they make standard with one actuator. Starting at 48 x 12 they are all dual actuator