The RD-6700 is a short cage model (SS), the RD-6703 is the longer cage style (GS) designed for triples. The "A" you are referring to is only for interchangeability of specific parts; its not really a part number. As far as I know, Shimano spec for both these 2 RDs is 28T cog max. The total capacity is what varies; 33T for the 6700 (designed for 2 x 10) and 39T for the 6703 (designed for 3x10). Of course, you can probably get away with using a 30T cog with the RD-6700 but you may have to turn the B screw all the way in.
Now onto the RD-6800. This is for 11 speed, where your old RD-6700 was 10 speed. This wont matter one bit. For the 6800 series, Shimano elected to add the SS and GS to the part number, which essentially mirrored the old 6700 and 6703 designations. Long story short, both versions of the 6800 are rated for 28T max cog. But again you can probably get them to work with a 30 T cog by adjusting the B Screw all the way in. The SS is rated fro 33T total capacity, whereas the GS is rated for 37T total capacity.
In closing, what literature are you referring to that specified your current RD-6700 is rated for a 30T max cog. Glad I don;t have that hill to climb.