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I can't test it right now, but they should stack.
Haste modifiers from external sources do not stack but at the end of WotLK I tested various things with PI:
A mage with IV got the haste increase from PI but with BL already up he didn't gain any additional benefit.
A shaman on the other hand still got PI's haste even with BL up!
So BL (external) + PI (own) should stack.