Heated grips are a waste of time.
I've ridden in -7ºC standing temperatures which, at motorway speeds with the wind chill, put it down to -35º.
I was wearing thick, waterproofed winter gloves and had heated grips on full blast - the inside of my hands were almost burning and certainly sweating (good idea in winter, eh)... but the backs of my fingers were still painfully cold and touching the levers felt like ice.
Large handguards or muffs to keep the wind off will be far more benficial than heated grips... but will also make your bike look stupid.