Very cool idea.
I would change the description a bit though because the way you have it now sounds like just a regular buff. I was going to bash this idea until I read that description.
Have you considered using existing mechanics like time bubble? You could have the creature go in time bubble for 2-3 rounds, then come back as buffed. I personally would change it to 3 and give a bigger buff. I like the "omg the big guy is coming soon!" effect.
There's also an insane difference between the two versions. Not only is upped version 50% cheaper, it's also twice as strong. You should do something about that.