From Halopedia, the Halo wiki
But there IS throwaway fluff. For example, we completely disregard in the games where the Plasma weapons are clearly unable to be headshot capable. Where we all know that is merely because of gameplay, and Plasma is more effective than UNSC Projectiles in that regard. So to this may merely be a thing added by Bungie just to make Sprint less than a thing they pulled out of their butt. Also, I believe that in the MJOLNIR armor, when you reach a certain speed, or bend yourself in some way, the armor will give you a warning, like, "WARNING: SPEED EXCEEDING RECOMMENDED PARAMETERS. WARNING: EXTREME PRESSURE ON ACHILLES TENDON." But would never not allow you to reach a certain speed.
Re: Your edit to MJOLNIR Mark V: But as I said in my summary --regardless of how fast he ran-- is that if the Safety Inhibitors didn't kick in, even when John was running that fast, than when would they kick in? If there were safety inhibitors then they probably would have kicked in at maybe 80 or 85.