I've used a [reed farm -> fermenter -> biogas engines -> transformers -> EU] setup in multiple worlds and it works just great.
Using a single 21x21 reed farm (with optimal fill pattern, 80%) 1 biofuel engine on a forestry harvester - produces more reed than you know what to do with.
The fermenter needs 1 biogas engine powering it for every ~25-ish engines that need biomass.
And If you want a REALLY OP setup with forestry - do it in an accelerated mystcraft age lol.