I have always understood the Encumbrance rules in SF-AD to be that a PC's maximum carrying capacity is equal to his STR in kilograms. So, typically, this would mean the weight that he can press overhead would be equal to his STR, and his deadlifting weight to be twice that amount.

If an average human STR is 45, then an average PC can deadlift 90kg/198lbs, and press 45kg/99lbs. This fits well into the average STR scores in the TSR D&D/AD&D scales for average strength scores, but when you get to SF-AD upper limit, 105 maximum, the strongest human PC can only deadlift 210kg/462lbs, and press 105kg/231lbs.

With AD&D setting the bar with an 18/00 Strength for an overhead press to be 470lbs/213.6kg, it seems that the SF-AD Encumbrance rules do not account for exceptional human Strength. This was also tempered by the rule that no character could lift more than twice his own bodyweight over his head.

If you double the SF-AD scale to account for the exceptional strength, you end up with average folks lifting 90kg/198lbs overhead, and deadlifting 180kg/396lbs, but this would be a bit overpowering, considering that an average human being of average fitness presses about 60% of their body weight in one push-up, and about 80% in a single pull-up. Supposedly, this put your overhead pressing weight at 70%. 

If we guage an "average" human male at about 80kg/176lbs, then the average his overhead pressing (70%) weight should be 56kg/123lbs, and his deadlifting weight at 112kg/246lbs... so the average human male should have a STR of 56-ish, and an exceptional human, of the same weight, would lift a maximum of 160kg/352 overhead, and deadlift 320kg/704lbs.

Has anyone come up with an "exceptional strength rule" that does not overpower the game?