Well, before the solid fuel nerf, I went through about 30 or so stacks of peat (and several of coke and coal) getting the temperature up to a decent level on a 27 block high pressure boiler. I also had about 6 peat farms going at one point, then once the boiler was up to temp, I dropped down to about 1 and a half peat farms running. I imagine now, you would need 3-4 peat farms running once the boiler got up to full temp.
Peat is definitely a good fuel to use with solid fuel boilers as you can easily mass produce it, "grows" faster than a tree farm, saves your coal for converting into coke/diamonds and doesn't need diamonds to craft the machines. Also since RP is out now, peat farms can easily be automated. (along with the necessary wheat farm, moistener and carpenter for producing bog earth)