Indeed only 20%. But since every civ with a strong eco bonus doesn't get treadmill crane, spanish are the best.
well aztec, hun, teut and maya got it; their eco seems stronger than the spainish one
Keep in mind that treadmill crane is 300f/200w you have to gather more with that eco boost. I don't think those civs will gather those additional resources in the same time window. The teuton/hun bonus is basically nothing with the low amount of houses/farms. mayan bonus might help you get food a bit faster and require 1-2 farms less, not good enough. Aztecs bonus hardest to judge, but looks too small.
Afaik you can't make mangs on wonder race, so no killing your allies.
You could do it about an hour earlier, then you could do both.