Campagnolo Record for less than Ultegra.
Campagnolo is used to varying extent on tandems, most often for shift/brake levers. I have Campy Super Record 11-speed levers, rear derailleur, cassette and chain. I really like having the 11 cogs, which surprisingly works fine with a triple.
The big disadvantage of Campy, of course, is that it is more expensive.
I've been looking to upgrade the worn 10 speed on my BMC half-bike to 11-speed, and shopped around for good prices on Campy. PBK was pretty good, but didn't have 12-29 cassette I wanted. Neither did Wiggle nor Total Cycling. So I googled Campy 12-29 cassette and found an amazing price at Slane Cycles, in Belfast, Northern Ireland (thus the UK).
The price on a complete Record group (brakes, crank, everything) at Slane is $1397. No sales tax, and free shipping. This compares to the price of Ultegra at Bike Tires Direct (typical U.S. supplier) of $1445.
(I know that tandem builders don't need the entire groupset, but I use it here as a proxy for the individual components).
From what I've seen on message boards, Slane has a good reputation. So I placed my order and I'll find out whether this is too good to be true.