Apples for apples, the 52x14 combination will be more efficient on a mechanical efficiency basis, probably about 2% more efficient. That would well and truly dispel any losses due to the minor additional weight. The main issue is not so much the size of the big ring on the front, but the small cogs on the back; the difference in chain rolling angles between a 14 and 12 is quite significant. There is an engineering paper on it somewhere, haven't looked at it for a while though.
You could always run a modified Dura Ace Cassette. Here's the tech docs for 7800 and 7900:
http://techdocs.shimano.com/media/techd ... 608753.pdfhttp://techdocs.shimano.com/media/techd ... 728250.pdfIf you remove the 12 and 13 rings (and might have to replace the 14 with an inner ring type with built in spacer), then you can add other rings on the back. I'd start with a 12-23 cassette so that you have an 18t in there, and then add a 24 and 25, or say 25 and your bail out gear or whatever-easy to change for each race depending on conditions. Miche make individual rings to do this.
The other advantage to starting at 14, is that your ratio's will be closer together where they matter. The % difference between 12-13-14 cogs is great that 14-15-16, etc. Going to 11 is crazy I reckon given the drivetrain efficiency losses and
For the record, I'm looking to build up a 44-30 or 44-33 crankset, with a 14-15-16-17-18-19-20-21-23-x (x=25, 27, 28 or 30, depends...) set-up for long audax/rando multi-day/week rides. I'm more interested in maximum efficiency and endurance over all-out speed for those though, I'm happy to tuck into an aero position and freewheel on steep descents, saving calories. I'll build my cassette up from Ultegra cassettes and Miche cogs. I don't mind that the set-up could weigh slightly more (rings and chain will be lighter, cogs will be heavier) because the power losses are much more significant. Also, I can happily run no front derailleur, cable and DT shifter on that set-up if things are pretty flat, and only change manually down to the bottom ring if I get to a decent hill.