Im running down the numbers and the IC2 fermenter isnt all that bad now that i look into it.
I looked into this way back and because of that i had a bias that it is extremly slow, however that is actually not that true in gregtech.
I managed to find one of my old threads about this way back:
http://forum.industrial-craft.net/index.php?page=Thread&threadID=11146&highlight=
The interesting thing here is that the eu value of biogas is actually much higher in GT5 U than using an industrial craft generator so each 62.5 seconds you would produce 1 bucket of biogas and 40000/64000 of this would be consumed to produce the next bucket. However that leaves 0.375 mb of biogas at 45 eu/mb or 16875 eu worth of biogas. So the output is actually 16875/(62.5*20)= 13.5 eu/tick worth of fuel. When you look into how much energy you get out of this it really gets interesting.
8 leaves is 1 biochaff which in turn transforms into 8 buckets of biogas if i remember correctly. Ofc a lot of this is consumed so in the end you end up with 3 buckets of biogas per 8 leaves or a wopping 135000 eu. That is a LOT for a bunch of leaves you can easily mass farm with a shear.
I will look into the numbers a bit more closely though to make sure its all still valid. However if the values is somewhat close to this it is actually an extremly viable source of energy production as the fermenters are the only thing you need to scale up since both macerator and canning machine work a lot faster than the fermenters. It wouldnt even be that hard to run a blast furnace of this.