Adding a leather grip will add around 8 to 10g in static weight. I added a Wilson leather to my racket and it increased the static weight by exactly 10g. If you don't mind the extra handle weight then its a great addition in terms of feel.
However, if you are sensitive to an extra bit of weight and are used to using more comfortable synthetic grips then you will notice the difference. I know I do. Whilst the feeling with leather is great, after a 4 hour session my wrist and hand were really aching. But if you don't mind that, then leather is the way to go for sure.
"Tennis is just chess with balls....zugzwang"