I just built up one of my frames with 9-speed STI and a 12-25 cassette, and am using a 1995 XT long-cage rear derailer (labeled "8-speed"). Shifting works very well, although I don't need the extra capacity for either total chainwrap (53/39 double crank) or large rear cogs. It's just the derailer that I had sitting around.
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"c" is not a unit that measures tire width