Islander - Are you sure ?
I don't have a good copy of the performance curves, but looking at a graph of some test stand data on an IO-550, BSFC seems to vary only 2-3% from Peak to 100 LOP with Gamis
Well, from eye-balling the graphs I thought I was sure that's what Continental's curves say. But now I've marked, measured and done the maths, turns out you're right.
I'm looking at their Operation Manual for IO-550A, B, C
dated October 1983 (Form No X30565) ... regrettably, Continental have chosen not to include this useful data in later editions of the manual!
The BSFC graph at 2300RPM/20.5"Hg shows 0.4165 lbs of fuel per BHP-Hr at peak EGT versus a best of 0.4085 at 35F LOP and 0.4095 at 25F LOP. That's approx 2% improvement at 35F LOP and 1.5% at 25F LOP.