As galio, Galio should be getting his build completed by now. This will give him a massive spike in his ability to peel for the allied carries. His damage output will be pretty high as well.

Galio's Ultimate R will still be handy during fights due to its knockup and low cooldown. He can quickly go into enemy territory to deward the area/set-up vision and then come back.

Level 16 isn't that big a spike for Galio. Although his Ultimate R cooldown is reduced and the cast range is increased, he won't have any incentive to use it for himself. As galio, Galio should be finished now. This will give it a massive peak in its ability to peel for the allied carts. Its damage production will be quite high as well. The Ultimate R of Galio will always be useful during the fighting due to its collapse and low cooling. It can quickly go to enemy territory to divert the vision of the area / set-up then return. Level 16 is not so great a peak for Galio. Although its Ultimate R cooldown is reduced and the range of casting is increased, it will have no incentive to use it for itself.