My very tentative guess is that for most people, there is substantial room to increase diligence. However, at the very top of the spectrum trying to work harder just causes each individual hour to be less efficient. Also note that diligence != hours worked, I am often more productive in a 7 hour work day than an 11 hour work day if the 7-hour one was better-planned.

However I am still pretty uncertain about this. I am pretty near the top end of the spectrum for diligence and trying to see if I can hack it a bit higher without getting burn-out or decreased efficiency.