My trim tab looks almost exactly like the one in the photo above. The trim tab itself follows the contour & outline of the stock aileron and is hinged on the top skin. The servo is mounted in the aileron just forward of the trim tab. It works well for the full speed and loading range of my RV-6.
The aileron stick force does change with speed in the RV, but it's really noticable from about 180mph to 210mph IAS. In my airplane, there is also a change in required aileron trim between those speeds.
Even with the aileron trim centered, the aileron stick force near Vne is much higher than at normal cruise airspeeds. For "fast" RV's (i.e. ones that can operate near Vne), I would guess that an actual trim tab would be better than the spring, but I haven't done a side-by-side comparison.
The one thing that's nice about the trim tab is that it has enough roll trim power for me to run full left-side heavy without an issue (i.e. pilot on the left, full left tank & empty right tank). I'm not saying I do that regularly, but the control power is there if I needed/wanted it for whatever reason.
I'll be out of town this week, but I can take some photos of my setup & post them if anyone's interested. Hope this helps!