My notes while testing this :
12 casts
21.51%
~1.6 sec cast
9142 haste
16 casts(flew off when 17th was nearly complete)
27.59%
~1.6 sec cast BUT it was chain casting it constantly now, rather than a little break after every cast
9142 haste
16 casts(flew off when 17th was nearly complete)
26.00%
~1.6 sec cast(fist cast was 2 seconds for some reason)
8502 haste
Testing base haste again(no pure haste gems only spell haste buff)
16 casts(flew off halfway through 17th)
26.00%
~1.6 sec cast(again, first cast 2 seconds, assume because it's not affected by the shammy haste)
8502 haste
My personal conclusion is that there's a haste breakpoint at around 26% orso, personally going to keep an eye on this thread and report if I have lower haste values or different results in raids with BL etc.