No... I actually used a combination of gears and item buffs to get my cspd to 1350. My cast time was still 1sec for these spells. Then I swapped my gear around to find the exact point in which I notice the cast time begin to increase.
Since going from 950 to 1100 decreased the cast time by roughly 1 sec, I'm sure increasing my cspd to 1350 would have reduced cast time to 0, if it wasn't capped.
maybe is capped or need certain -cast speed% , i dont have alch, but i got hw, imperial is the slowest skill but at very high cast speed beat meteor wich is slow than imperial, but only works with mirage -20%, bish -50% and mins, if not meteor always win.