I have a similar issue, but it only happens when I've been on the bike for about thirty minutes already before encountering heavy traffic. In CA I can lane split, and my hands start to get numb after a lot of lane splitting (I'm talking miles of constant lane splitting). I can't take my hands off the bars because I'm going between cars and need to be able to react quickly. When there's no traffic on the highway, I can remove my right hand for short periods of time and left hand for long periods of time.
That's pretty much the only time I have issues. But I still have the stock grips and am thinking different grips may help a little bit. On the open road I have no numbing problem.
2012 Triumph Thruxton 900