I'd like to know how you tested the recasts. Using Windower's recast plugin, as far as I've been told, is not an accurate way to do so. Personally, I play nin almost exclusively and have fast-cast on both anwig salade and mirke wardecors. Although I change to turban midcast on both spells, I do keep mirke for the recast reduction (small as it may be.)
The reason I ask is that with regular marches, haste spell, and my gear, I've observed Utsusemi: NI recasts as low as 20 seconds (according to the windower recast plugin). I had thought that all spells have a hard cap on recast reduction at 50% and assumed it was an inaccuracy in the plugin. I'd like to test it again with a better way of determining recast.
Use your spell list.
Like you said, windower's recast is not accurate enough for testing. I use the spell menu for accurate recast time, and it hasn't failed me yet~
Since 50% is the recast cap, and it's rounded down, 22 seconds should be the minimum for Utsu: Ni. I've heard accounts of a 21 second recast timer, but I've never seen it myself.
Good find... I erased my post because I was being stupid and thinking of the data being server side for some dumb reason.