Upon further testing, 5 MFR Boilers (at 400mb/t steam each) are enough to push a full sized BR Turbine to just shy of 1800RPM. With my setup of a single Lava Fabricator and a single Fluid Transposer, I have a steady flow of lava buckets to fuel three turbines (15 boilers). In my Infinity world, these are putting out 28krf/t each.
If I were to add more boilers, I may need another fueling system. I'm not sure if Augments on the Transposer could increase the bucket fill speed enough to keep up, but even so, I may need another Lava Fabricator to fuel it. I won't know until I have need for more power, but for now, I have way more than I need.