How to get a lower gearing
The smallest chainring that will fit on a compact double 110 BCD is 33t and they are quite hard to find ( Only TA or custom chainwheel manufacturers make them). 

IMHO, what would be a true triple replacement would be something like a 11-30 with a 30/48. 

Unfortunately, the 94 BCD cranks are almost all old MTB triples. 

The problem with putting a large range cassette on that bike is compatibility with the Ultegra 11 spd shifters. I know Shimano 9spd is interchangable between road and mtn, but I'm pretty sure that is not the case 11 speed deraillers. Shimano does sell an 11-32 with that groupset Ultegra 6800, so they must think the medium cage derailler can deal with it. 

http://www.chainreactioncycles.com/us/en/shimano-ultegra-6800-11-speed-road-cassette/rp-prod108686