what are the chances that ivy bridge will use higher memory speeds?
The only difference is the IB default, like the current SB-E, is DDR3-1600 vs SB's DDR3-1333. As far as maximum, DDR3-2600 or faster. Folks get tunnel; vision over frequency I on the other hand look at 'real' speed which includes CAS Timings which are frankly more important or at minimum equally important to performance.
My best 'guess' is DDR3-1866 will be the IB 'sweet-spot' with CAS 9.
Nice chart, note Frequency and CAS: