http://feed-the-beast.wikia.com/wiki/Magmatic_Engine
"When the engine is producing 4 MJ/t, one lava cell or bucket will produce 18,000 MJ, a Thermal Generator, will make 30,000 EU per bucket."
2.5:1 conversion ratio is 12,000 MJ from 30,000 EU - worse it's 10,000 MJ with 3:1
by converting...