I'm surprised that two 11x11's can support that many boilers.
I don't know that my 9x9 could support anymore than say... 6-7~ of them. I'm not actually sure, actually. That's also not considering boilers running on the wood... hmm...
I have those two 11x11 and I have 2 Steve's carts treefarms, the MFR treefarms completely blow the Steve's cart farms away. I'm currently at over 1 million surplus charcoal, 2 9x9 railcraft tanks are constantly full with biofuel, so it can easily handle more. MFR farms provide more than adequate resources, being able to process and deliver them as fast as you get them is key.
If you have a sufficient production of fertilizer, a 1x1 MFR treefarm can power 5 solid fueled boilers. Certainly more, its just that I only built 5. Thousands of extra wood every day.
The saplings, I don't use at all, so that's another few boilers.
I run half biofuel and half charcoal but I don't use the saplings to make biomass, I use the shear leaves function to get leaves to make plantballs and make them into compressed plants, I then use those to make the biomass.
Here's a video I made last week when I got my system up and running.