No, a RD-5800-GS has a minimum large cog size of 28T. RD-5800-GS (shimano.com). But I am using a 12-25 cassette, forcing me to have the B screw all the way out. I can get the two RD pulleys pretty close to vertical but not perfectly vertical. It shifts fine so I am OK with that. Next year, before I replace the RD cable, I am also installing a RD-6800-SS and a 11-28 cassette, so maybe then I can get the two RD pulleys perfectly vertical then.