Well, I've just played a game which is perfect to reflect these thoughts of mine.
Cataract - Regulus, Oak and Unclean Beast (me) versus Rook, Erebus and Unclean Beast.
Thanks to an excellent Rook player, they managed to lock us early game (before they even hit level 4-5) and destroyed middle towers + archer towers. It looked like a game over.
We were pushing really hard and we managed to hold them somehow, but it was impossible to hold a single flag for longer than a minute - they were having a real advantage. Still, we did prevent them from attacking our towers, although they were close once again.
I was patiently waiting, carefully killng creeps for XP (we were behind......) and helping in the middle when they would push.
It was not looking good. They upgraded to catapults immediately, while we were trying to get Priests.
It was good as done.
We did the necessary Citadel upgrades and tried to defend and push them back.
When I managed my first Rook kill, things started going easier. Without his tower support, they had to flee, we took over flags and were getting amazing amounts of XP/gold for their creeps.
Rook couldn't get out of his base for a second - I'd use Serpent Sword, spit, foul grasp, run for him, spit, ewwww. I managed to kill him two more times.
Now, we were stacking enormous amounts of gold for killing their creeps (while they had nothing to feed on), so I went for Mage Slayer. To make long story short - the early advantage they had, which was supported by catapults, turned to be their disadvantage, because they could not push through us again, and we were sweeping away their creep waves.
Game became a stalemate.
One death of their DG changed the tide; we were able to rush in and nuke their base.
But it took a while to get to the point of actually being able to do it, and most of DGs were level 20.
Now, when I went to check the Citadel for the last time, believe it or not - almost every single upgrade was maxed out, except experience bonus - not a single upgrade.
I bet I'd see at least one upgrade if we were not capped at level 20.
But I might be wrong.