Just fitted this exact combo for some serious climbing. Works well, but I can't really tell the difference between 27T and 28T.

Just be aware that if you are using the older generation of rear derailleur (e.g. Ultegra 6600, 105 5600), they are not designed to work with a 28T cog. It will fit, but to avoid the derailleur pulley contacting the cog you have to have the B screw wound all the way in, which means that in the higher gears the shifting is not as good as it could be. Still works okay, though.

If you are using the current generation Shimano rear derailleur (e.g. Ultegra 6700, 105 5700) then no problem since they are designed to work with a 28T cog.