An overclocked 270X can have the potential to theoretically come close to the performance of a 280X. My 270X can theoretically achieve 3.02 teraflops where the standard 270X is 2.6 teraflops, and a 280X is 3.3 or 3.4 teraflops or something.
An overclocked 270 non-X can have the potential to reach performance of stock 270X.