The first lifebloom you cast after SOTF will benefit from the buff. So in your example: the target had 1 stack, you used swiftmend and gained the SoTF buff, you lifebloom again for stack 2. He now has a 100% hasted 2 stack LB on him.
Now if you cast a 3rd stack of LB on the target....I believe you lose the 100% haste buff from the 2 stack and just go to a normal haste 3 stack. I'm not 100% sure (been ages since I paid attention) - but if you don't get a response in the meantime, I'll check it out tonight at raid :P
(I personally just make sure the hasted LB is either the 3rd stack or refreshing a 3rd stack) - although really with how much tanks heal nowadays - ur better off using the SoTF buff on WG or a Rejuv anyhow