The differences from 2007 to 2008 are significant, but not quantum.
In terms of sail size range to board size (model) range, although the range for the larger sail size for each model has increeased, the new boards don't change the sail size sweetspot that much. So (basically) where you were happy with iS111 previously, in 2008 you would take a similar sized model and just enjoy the improved performance over a similar sail size range.
As with the previous other discussion/s on these new iSonics, for 7.8m and 6.8m use, it's perfectly suitable to use 2008 iS111, although for guys preferring a "smaller" board for these conditions, the 2008 iS101 becomes even more practical.
Cheers ~ Ian