In the final 1.4.7 versions there is a bug with the multifarm when you combine a farm type that involves irrigation holes together with a farm type without irrigation holes on a 'managed' farm. The farm will end up confused about whether or not it needs irrigation holes and where to put them, resulting in it placing and filling and emptying and re-digging them chaotically. It especially likes to get stuck in a state where it constantly spends 1 water bucket per action to fill in a hole, over and over, for long periods of time. This is what people see when they report that even multiple aqueous accumulators are enough to keep up with the farm's water drain; it's not by design, it's a bug.
In your case, the mushroom farm is the section without irrigation requirements that's messing up your farm. Considering how little use mushrooms actually have, I highly recommend you ditch it in favor of only vegetable and wheat farm sections, which all require irrigation and as such should not trigger the bug. But if you want to be extra secure, make it a manual farm instead. You'll even be able to gain a few more blocks of farmland by intelligently designing the irrigation (especially by taking advantage of the possibility of surrounding the farmland by a ring of water).