Well, perhaps my analogy was incorrect. Here's a better one. Instead of Gohan topping out in 4th gear like everyone else, he has a 5th gear.
Super Saiyan 2 Gohan (Rage Boost) 250
Super Perfect Cell 240
Super Saiyan 2 Gohan 200
Perfect Cell 150
So Gohan has another notch. He can't kick it into overdrive with his rage boost.
And to address the matter at hand, Gohan's rage boost are so large when he is younger because he is so inexperienced and so untrained. By the time he is facing Cell, he has trained many, many years and has reached most of his potential on his own, without the need for a rage boost. The rage boost essentially unlocks his dormant power, so the more trained he is, the less dormant power would be accessible.