I'm very happy with the harvest champion boost. Two trades back to back wiped out my corn and wheat, so I decided to try it out. I started with 42 energy and harvested 20 wheat and 20 corn, ended up with 29 energy, so it appears to boost the energy return too. (13 energy to harvest 40 crops!) Before collecting I had 68 wheat in inventory, after I was maxed. 25 corn in inventory became 67. The boost lasts 3 hours, so I've planted more corn.
Just make sure crops are ready to harvest before activating the boost.
My goal was to try to hoard runes for the Ali and the kiln, but after 10-12 trades and zero gold runes, I'm thinking the harvest boosts will be helpful at least. They will certainly make the strawberries and glimmer bulbs easier to replenish after doing the energy trade in the exchange.