I had a hell of a time with my concord 30 gallon pots. (cutting a total of 6 holes)
The pilot hole was the easy part, contrary to what others are saying. I used a cobalt bit and some tapmagic. 1 bit easily cut through all 6 pilot holes. Go fairly slow and give some pressure. Center punching may work harden the surface and make it harder to cut through. I did not center punch anything.
My issue was burning through a step bit in 2 holes. Then I bought a cobalt step bit which was better, but still a royal PITA. It got to a point where the bit would cut to a step, then not cut anything no matter what I did...so when it stopped cutting, I took a dremel and went around the edges of the hole, just enough to remove a tiny bit of material. Then the bit would grab again and cut, at least to the next step. Weird.