How can you continue to ignore monk completely?

[Shurrikhan]

MNK's speed at GL3 is [virtually] the same as NIN's Huton actually. (Actually, GL is like 0.01~0.02 slower, which is strange because the weaponskill recast reduction are both 15% but a NIN with 1139 Skill Speed vs a MNK with 1234 Skill Speed still has that slightly shorter recast, so I guess that's chalked up to stat weights per job.)

I figure it compounds?

If GL1 shaves 5% recast time, so from 2.5 to 2.375, GL2 would reduces it to 2.256, and GL3 it to 2.143.

Huton shaves off a straight 15%, granting an immediate 2.125.

Their flat contribution is reduced at lower base GCDs. (The percentile, of course, remains unchanged, as does their actual contribution.)

EDIT: Or apparently they're both identical, as GL's reductions work additively, rather than compounding. Both still reduce the GCD by a true percentage, though, so the seconds reduced will be less if the original GCD is less (due to Skill Speed).

[SunnyHirose]

It compounds.

GL1 shaves 5% recast time, so from 2.5 to 2.375.
GL2 reduces it to 2.256.
GL3 reduces it to 2.143.

Unfortunately I can't even pretend to know the answer to the original question, but if you take a no-SS monk to a dummy you will observe 2.50 -> 2.37(5) -> 2.25 -> 2.12(5).