From what I can gather, throwing or improvising weapons is based off strength, so at 20 strength you can lift/throw up to 80kg (or a standard humanoid creature).

Does having any carry capacity bonus (such as the human versatility of bull's strength) have any affect on what objects can be thrown or used as improvised weapons?

If I can have a guess at the math, human versatility should increase carry capacity by 25%, and bull's strength increases it by 100%. Does this mean humans can throw up to 100kg? Will bull's strength push this to 160kg? (or 200kg if it stacks?)