Note that the multiblock farms behave differently than the old forestry farms.
The old farms consumed a constant stream of MJ, scanning every single block in their range one by one for work to do, until they found something, and then performed the task on said block. The more MJ you gave them, the faster this block scanning happened, but since plants need time to grow, the largest part of your power input was wasted.
The new farms only consume resources (water, fertilizer and power) when they are actually processing something (chopping down a tree for example). While idle-scanning, they only use something like 1 MJ every couple seconds; the same thing a Railcraft rolling machine does when constantly powered but sitting idle.
Therefore the best idea is to buffer the MJ output of your engine. Instead of connecting the engine to the gearbox, connect a redstone energy cell to the gearbox and connect the engine to the cell. That way you can use a small engine to slowly build up energy in the cell while the farm idles, and let it consume energy in maximum-speed bursts whenever it finds something to do.
You can also simply use conduits if energy cells are too expensive; but a conduit will only buffer 1000 MJ. Should be easily enough for farms that only replace one block at a time, but for tree farms specifically it might not be enough since they have to chop down trees made of many blocks at once. Granted, the farm itself also has a small internal buffer, so together with the conduit it might be enough for small trees like common apple oaks. Would likely vary a lot due to the random nature of growth, though. Two trees growing in quick succession will deplete the buffer, while no tree growths for a while will saturate the conduit and waste the engine's output. Still less wastage and more buffer than directly connecting the engine, though.
