If the player's Endurance stat is the most extreme (highest or lowest), Doc Mitchell will either say "I guess that explains how you're still alive. You're built solid as an oak." (for highest) or "I just don't get it. A stiff breeze'd tear you in two but a couple of bullets and you're right as rain." (For lowest)
An Endurance of 9 will guarantee you the ability to get every implant in the game, the 9 implants by Dr. Usanagi as well as those offered as Perks. This level of Endurance can be reached by initial S.P.E.C.I.A.L. allocation or raising it by the Intense Training Perk or the raise awarded by finishing the Lonesome Road add-on.
The Endurance Implant by Dr. Usanagi *WILL NOT* raise the number of possible implants, although it counts as a permanent raise of the base Endurance score for all other practical purposes, no matter if chosen first.