I understand that putting on an overgrip on the handle reduces the balance point of a racket making it more headlight, but does that mean it also decreases the momentum of the swing, hence less power in the shots?
No. Adding an overgrip does NOT make a racket more head-light - it makes it more handle-heavy. The mass of the head is still exactly the same, so the racket will still swing exactly the same, and still hit with exactly the same force. Remember, the balance point will be lowered, but the overall weight has increased. The only way to make a racket more head-light, would be to remove weight from the head (short of sanding off the paint, this is impossible). On the other hand, it IS possible to make a racket more head-heavy, by adding weight to the frame (eg. lead tape).
I'm not sure I agree.... Head 'weight' (light/even/heavy) is a relative value, really all it's describing is the balance, so making the handle of a racquet heavier is, in effect, making the racquet head-lighter. But in this scenario the weight of an overgrip is so small that I doubt it'll make any noticable difference, perhaps measure the balance point before/after if you're really interested. (worth noting that the size of the grip can affect how the balance feels even though the balance point hasn't changed)