I can't test at this exact moment but I can tell you that using a config 14 IC and feeding 621 mB/s of lava produces just under 40k mB/s of superheated steam. I'm not sure where the efficiency hit is calculated since it isn't reflected on the scanner (implying any efficiency hit from maintenance is compounded and may end up producing regular steam if operating near the edge). If I'm aiming for 40 k mB/s of superheated steam is a config 14 IC optimal? That should be a 21% efficiency hit for a threshold reduction of 525 mB/s.
Also I calculated needing 632.9 mB/s of lava for that output but experimentally determined roughly 621.5 mB/s. 500 / (1 - (1.5*14)) = 632.9