I had the extreme pleasure of talking to Jimmy's mechanic a few years ago; a fellow named David Sims. He is currently managing the Risi Competizione sports car team in the American Le Mans series. David was there at Hockenheim in 1968.

We talked a while about drivers and how Jimmy might have done if he'd gotten to drive some of the modern F1 cars.
There's a great story about how Jimmy asked for a slight setup change before a race, then went out on track and scored a dominant win. After collecting the trophy, interviewing the press, etc., etc. he came back into the garage and approached his mechanic. "You know that change I asked you to make? It was worse. Please put it back." I asked David about this story and he smiled. "That was Jim!".