I had just started last year, and was a bit disappointed : the short duration of the cloaking device didn't really allow to "play predator". Does it increase significantly during gameplay ?
Nope, the suit powers aren't upgradable and you stay with the same amount of energy until the end of the game. You just learn to be more efficient with the suit: you are supposed to uncloak every now and then, just enough for the suit's energy to be replenished. Also, if you operate a weapon while being cloaked you'll lose all remaining energy: you can counter this by uncloaking for a second to shoot someone down before cloaking again (and you better reposition yourself after that because the enemy will continue to fire in your direction).
It takes a while getting used to it, but it's what matters the most in Delta difficulty (aside from being stealthy). Having said that, if the available energy pool was bigger, there would be serious balancing problems with the game; you're already cheating with that suit.