I found that the hand guards still don't keep my hands warm at high speeds on cold days. Above 60 mph it's hard to tell the heated grips are working, the only way I can tell they are working at high speeds is to turn them off.
My solution was to buy the Gerbings glove liners. Now I get heat on the top of my hands where I need it and my fingertips stay warm. I can turn the heat up as needed also.
Since the heated grips quit working every once in a while I figure the glove liners are less expensive to replace and they actually work better. Combined with the Gerbings jacket liner, I now stay much warmer overall. I also found some lightly lined waterproof gloves that I bought in a larger size to accomodate the glove liners so I'm not fighting all of that insulation under my hand trying to grip the hand grip. My hand doesn't cramp from holding the hand grip with all that insulation now.