Some of the pro-xp upgrade comments here don't make sense.
Look at the table again. Forget about whether or not the other side buys priest upgrades, etc. If you buy the first xp upgrade, you gain no level advantage through out the course of the game by virtue of having that upgrade. You will get levels faster than your opponents, but you will not be more than one level above them at any given time by virtue of the upgrade. If you have the second xp upgrade, you get one full level advantage, and that's going to be at level 11 or later (most likely later, since you're not getting xp at level one).
OK, I understand you can get the xp flag on cataract and add that to xp. But you can still get the benefit of that without buying xp 1. And if you hold the xp flag the whole time and have the spare gold to buy xp1, you would probably have won the game anyway regardless of whether you actually bought xp1 or not.
I understand that if the enemy buys priests early, you can farm them. But you get most of this benefit even without xp1.
Another way of stating it is: if you are two or more levels above your opponents, it is certainly not because you bought xp upgrades. It's because your team is better than theirs. They are likely feeding you with troops and kills. Your level advantage is not caused by the xp upgrades. Had you not gotten the upgrades, you still would be outleveling them by almost the same amount. You probably could have bought the death penalty mitigation upgrade instead of xp and still won.