Am I the only one using the Hobbyst Steam Engine for the Forestry tree farm? I make steam, anyway, and steam is easy to transport. It's slow, but balancing with the growth rate of the trees, it seems about right. And, I think it's cheaper than many other options. I didn't do the math, but it feels way cheaper than the Ender IO Stirling Engine. (my farm is not the maximum size and there may be ways of growing trees faster, that I'm not aware of, but there is also the Commercial Engine, for this situation)
And it doesn't explode! (if you use steam, you don't need water)