If your derailleur is the SS version (rather than GS, it will be stamped on the back of the derailleur, something along the lines of I-RD7800SS) then there are two things you need to think about; max size and capacity. Technically the max size for this derailleur is a 27t but most Shimano derailleurs will work well over their max. There might be an issue depending on your frame and how close the dropout places the derailleur to the cassette, but it's unlikely.
Second is the capacity of 29t. If you're running 39/53 chainring then your needed capacity is (53-39) + (28-11) = 14 + 17 = 31
Again, this is outside the stated. If you size the chain based off the big/big (the way Shimano recommends) then you may have an issue with the derailleur not being able to tension the chain in the 39/11. Unless you're a chronic cross-chainer this probably isn't much of an issue.
So short answer - a 12/27 6700 cassette will be awesome.
a 11/28 6700 cassette will likely work at 99.9% performance.