I think 160mB steam per 1mB of water is fine, so long as hU's could be consumed in non-multiples of 100hU/t, which was my main concern. For example, it's not really possible to make coal powered steam power plants right now, since solid heat generators output 20hU/t, but only 4 can connect to a single steam generator (unless I'm missing some sort of heat distribution block, which I may be). Fluid heat generators also generate 16/32/64 hU/t.
As an alternative suggestion, perhaps allow the steam generator to keep track of fractional mB?
P.S. I checked the railcraft numbers to compare to ic2 steam (numbers pulled from here http://ftbwiki.org/Steam_Boiler_Statistics). A 2x2x2 LP boiler uses the equivalent of 5 furnaces of fuel at full heat to generate 80 steam/t which can be used to generate 50 EU/t (using railcraft steam turbine), the same as the 100hU/t equivalent from solid heat generators. Good on ya .