Kryptonite are plenty strong, they are a good deterent, but not theft proof by any means. I've talked with people who's job was to cut bike locks. It was generally something like cables, 15 seconds. Offbrand u-locks 45 seconds and 1/3rd of a cutting wheel, name brands minute in a half and a cutting wheel. Then the "new york style" heavy ulocks were on the order of 5 minutes and 3 wheels.

Certainly there are bigger cutting discs, if you are willing to make a bunch of noise and sparks you can cut them more quickly.