i have done some test on local environment one month ago.
a pal equip with a phoenix shield which has 100% change to cast firestorm when striking, use zeal to attack.
u know, firestorm has 0.6s(15 frame) casting delay, and zeal attack speed is 4 frame per hit.
the result i observed is
zeal attack is not interrupt by firestorm and keep it orginal rate - 4 frame per hit.
firestorm was cast by each attack without affact from casting delay.
15 frame cast delay is applied during each complete zeal attack.
in another word, the speed of zeal attack changed from 8/4/4 to 15/4/4
Which means a wolf would be screwed royally, as Bliz decided to pander to pallies and make Zeal uninterruptible. We'd get one hit at buku fpa, then interrupted by Firestorm, assuming it operated the same way. All in all, I'll stick to my Atma's/Drac's