Yes, outside of the initial high-C region, energy storage is essentially linear (or, C(V) goes as 1 / V).
Which goes to show, if you need bulk energy storage, and it has to be ceramic, you're better off with C0G, which has constant C and therefore quadratic E(V). This isn't unreasonable for high voltages (the energy density is better than electrolytics), but is rather impractical in cost!
Tim