I've tried this out on Dynamis/Limbus mobs and 1 short bird meritpo (No KA gear or Dunes equipped). My AF's were doing about 1/2~2/3 the dmg with FW active, and I was getting 37%-40% TP return. I only did one DK but it wasn't too hot.
Keeping it active seemed to take more time to get to 100%. Even though there is a TP boost, keeping it active doesn't seem worthwhile at this time. More testing needs to be done though. Possibly with full KA merits and a fair amount of +KA gear, the TP gain rate may be comparable.
Peace,
Shmed