Here are some AB casting times with different stacks and other stuff.

AB = Arcane Blast stack amount
TW = Time Warp
WH = Witching Hourglass heroic (+1710 haste proc)

Gear = +15.08% haste (1502 haste rating)

0AB = 1.74
1AB = 1.65
2AB = 1.56
3AB = 1.48
4AB = 1.39
4AB+WH = 1.24

0AB+TW = 1.34
1AB+TW = 1.27
2AB+TW = 1.2
3AB+TW = 1.14
4AB+TW = 1.07
4AB+TW+WH = 0.955

As we can see here, even though the trinket is awesome, at certain point the proc just goes useless.
Tooltip shows AB casting time 0.955 at fastest, but if it is 1.000 sec actually, then that trinket proc gives only 0.07 sec haste, which is nothing.