Per the topic, outside of Hulk and Superman. I cant really think of anyone, since this is specific to Superman in both character and plot.
I would wager Photon (Genis-Vell) is another, for different reasons.
In Photon you have a character that is not consciously holding back, but consciously unable to maximize his potential. As his very being is tied to the beginning and the end of the universe. He constantly draws power, and experiences his life at every a point of time through the time stream. With Photon knowing that his very being will provoke the collapse of the Multiverse, his cosmic awareness constantly tuned to find a solution to his power. Both instances creates a situation where his application of his power is diminished, due to his focus being thinned out.
If Photon did not care for the collapse of everything, and focused his attention in the present, we know that his capacity would go from matching the Sentry all the way up to killing Eternity…which is a massive gap.