I put headed grips on my VN750 4 years ago and did all the research you are currently doing (I posted results here at that time). Based on my research, HotGrips
makes the best heated grip elements, but I did not care for the style of them nor the HotGrip variable controller.
Warm and Safe
makes the best heat-troller Heat-trollers are pulse width modulator based, very efficient, and offer the best output control. The Warm and Safe heat-troller is also rebranded and available for other vendors (i.e., RevIt). Warm and Safe also sells a heated grip kit that includes Symtec elements.
Symtec makes a specific heating element for the right and left grip. The left pad runs a bit hotter to compensate for heat loss through contact with the metal handle bar. The right pad is mounted on the plastic throttle control tube, so it does not loose heat as fast.
I went with this kit over the HotGrips so that I could have more choices over the grips, and the option to switch grips the future without having to buy another set of heaters.
I could not be happier with the results. I should note that I don't ride when the temperatures go below freezing (which is rare here in Portland, OR). Also, I do not own a pair of winter gloves. I wear my full gauntlet spring/fall gloves and rely on the heated grips for the rest.
I don't think that I've ever had had the controller past the 70% of its total output. Beyond that setting I would need to switch to oven mitts. One note that it takes 3-5 minutes to get this setup running toasty when it is cold out and they cool off FAST.