I just did some testing on the test server with boost spells at level 95.
With light arts, capped enhancing merits and every piece of gear that has enhancing skill on it that WHM can wear (other than the Ix'MNK cape which has always managed to elude me) I had 460 enhancing skill.
Boost spells all increased the attribute by +21.
Maybe by 99 cap if they add another piece of enhancing skill gear, or let us merit it higher we can cap that out at +25.