he's adding 6 teeth in back and lost 3 in front, for a net gain of 3 teeth. his ideal chain should be 2 links longer than the ideal chain length for his original setup.

if the current FD is handling the 50-34 fine now, he's good to go there. his new rear derailleur needs to be able to handle (50-34) + (34-11) == 39T total difference between big+big and little+little? thats pretty extreme. me, I would loose that 11T and try to find a 13-34 or similar.

me, I would likely have been looking at a Ultegra 52-39-30 triple crank, and a narrower range cassette, like a 13-26. but the triple very well might have required a new FD -and- RD, and there are no duraace triples.