I don't really see the situation given as a contradiction. I know of individuals that fit this description and you seem to point to that idea in the post. I guess I'd ask what are you looking to adjust to, what the player wanted their character to be?
There are different kinds of athletes out there. The scientist might be able to carry a bunch more than the athlete, but, as you pointed out, isn't that good at doing much else when it comes to brute force. I know a few marathon runners than can't stand to carry a heavy pack. My out-of-shape large frame can easily carry the heavy pack. My friend will be able to outrun me there, but I'll be able to walk to the distance with the pack with the same ease.
As for fine tuning, I'd allow a wide range, but impose suggested limitations. You give suggestions for tweaks and how common such a tweak is.
It gives the end user the ability to adjust as they want, but not without warning that they might want to reconsider the concept if they are having to tweak this much.