I've been using RD-5700 GS / RD-6700 GS with SRAM 1070 32t cassette for years. With SS I can go up to 30t no problem. If I remember right, when I did a similar research some time ago, someone explained the reason why Shimano only spec'd up to 30t cog is because that was (or is) their largest 10 speed road cassette at that time.