There is nothing quite like seeing yourself from another perspective! I can well remember my first, 10th and 50th revelations by video. It really helps.
I agree with everything that Roger said. And, being female, I can't help adding one more thing. In the techno-speak of windsurfing, we talk about the "center of effort" in the sail (the focus point of the sail's "power") and the "center of lateral resistance" (the focus of sideways resistance). When sailing in non-planing mode, and/or with a centerboard down, you go straight by having the CE over the CLR, to steer downwind the CE is leaned forward of the CLR, and to steer upwind the CE is moved aft of the CLR. Basic stuff.
Once you get up and planing, and without a centerboard, the sides of the board and the fin become the CLR and are thus farther back on the board than they were when the whole board was in the water. A common problem is that sailors progressing through this stage are used to thinking of sheeting in as being what increases the speed and the transition to planing. However, one must still remember to balance the CE over the newly farther back CLR. If you don't begin to rake the rig back as you pick up speed, you end up sort of balancing forces but at the expense of ease of sailing.
By moving the mastfoot back, it will help balance the CE and make the sailing more comfortable and easy. You will plane more easily, and be able to lean back against the pull of the rig more effortlessly. Additionally, the mastfoot pressure will be more effective as Roger has pointed out so well.
Move the mast back a bit at a time so that the differences won't be too extreme, and so that you can really begin to feel the difference.
keep up the good work,