Well we know for a fact that if he fought either individually without shifts he would win, perhaps due to more stamina. But as far as power goes, they seem to be equal based off what's been shown.That makes a huge difference. Just imagine Luffy fighting Zoro and Sanji who are clearly weaker than him in shifts. He's not getting past second round. When Jack, on the other hand, didn't even seemed to get tired even after ten rounds which is a clear sign of superiority. They're not necessarily far from Jack but they're nowhere near being his equals. The chances of Jack conserving his strength are high given the fact that he lasted whole five days without taking rest.
Oda is a tease, Jack probably has a hybrid form that he never used in his fight with the dukes.