Having both built the Pyramids and having not built them, I like to not build them and use the GA in late MA when Im fighting my first war. I find you usually get to Mil Trad with the Maya at that time (500 AD)and thats the first time you can actually start increasing your pop/land dramatically. I dont know if pop boost from 1500 BC to 750 AD (steam power, rails, and electricity for irrigation everywhere) is worth the tradeoff in land score you get on the first 100 turns. Maybe if I had enough settler factory spots, and a huge land area to cover, the Pyramid shield cost would work better. The GA is nice but minimal in the long run, and better used to slaughter the hundreds of units a deity AI throws at you.
With the Iro I hope to grab ToA and the Pyramids when I start my conquest, preferably around 200 BC with accumulated chariots to upgrade.
On another note, what about razing cities? Im getting into the conquest phase here, and keeping the AI cities, which usually have markets, aquas, and granaries and are OCP spaced, seems the better option if you can win a war in ten turns, eliminating the AI. You retain the (soon to be happy) pop, and just have to deal with the culture flippage until you eliminate the AI. At first I was always razing cities, but Ive gone away from that and position cavalry between every few cities to take them back. I think this works a bit better, but havent heard it discussed much.