I'll summarize my opinions from cruising Slowtwitch and Timetrialling.uk:
1. Bend radius when keeping a good chain line and using a slightly larger cog in the back reduces losses.
2. Around here, you go downhill at some point. Bigger front ring, no spinout downhill. Confident even power downhill without bouncing on the saddle at 110rpm.
3. One more tooth up front means less than a tooth in the back. So take some of the bigger front rings with a grain of salt.
I'm on a 54T and 12-25. Works for most of what I roll. My cadence range comfort wise is about 85 to 105 for a TT. That gets me 14 to 37mph of range. A 56T would be 15 to 38.5. Not much difference.