on my cc-bikes i use 42x 12-25 or 11-23, depends on the track. i ride campagnolo 10 speed. when i rode shimano, i had 12-27, but i didn't need the 27 one single time. because the 12 cog is necessary, i don't use 13-26, and take a 12-26 and a 13-26 cassette and make a 12-26 out of it is way too expensive(considering that i need at least 6 pairs of wheels at the races...)
i love racing with a single chainring setup, you don't have to think abuot it during the race, and when i had 39-46, i always rode on 46, to fix the chain better. so i could no more ride with high cadence(lance-style...)
and on this homepage it is important to say that single chainring is lighter(i use spooky carbon disks left and right of the chainring).
for my right brake lever, i took a campagnolo-lever and took away the "shifting mechanism", this is lighter than the time trial lever you can buy at campa.
my friend rides shimano, he took a 7800-lever, took away the shifting parts of it, this is over 50 gr. lighter(those things don't make sense if you have to buy the parts, but if you get it from the sponsor, make it and get your bike lighter!)