One thing many people fail to understand is that individuals who are talented at one thing are often talented at many things—especially athletes. People like Mo Farah are incredibly athletic, and would perform well in a variety of sports. If someone like him really tried to hammer out the 100m, he would be able to run better than 99.9% of the planet. He might not be winning anything at the professional level but he would still be extremely good at it.
However, the question is more about who would do better in a different discipline. An elite distance runner could still probably be in the top 99% of sprinters on the planet. I have extreme doubts that the same could be said about a large, powerful, pure 100m specialist.