They are cut slightly lower than a lot of other skates, also baking them will make a huge difference. Graf skates can be baked multiple times so ask your dealer if you can bake them before buying. We do that for customers in our shop all the time. They'll close around the ankles a lot more than they do out of the box and should provide more than enough support.
I had a pair of G5's before buying the G75's. The G75's have more ankle support in general compared to the G5's. And as jsykes they're cut lower and baking them will help too. If you're comfortable in the skates other than the ankle support, maybe taping your ankles is an option.