Your bike likely doesn't fit you properly, or you are not using your abs as supposed to for correct body geometry (too much weight down and forward). Unless you are cycling 300kms with a perfectly fitting bike, I would see if the stem could be raised up, changed to a higher rise degree, or fork changed for one with a longer steer tube. I work in a custom fit bike shop, and this would be my 2 cents based on experience. Grips are the LAST thing to look at, once the fit is perfect and you can prove the numb hands problem still exists with perfect handlebar height.