Si mal no recuerdo, tome suficiente SPS para llegar a 2.17 de GCD con comida encima. Con ese GCD llegue a una rotacion aceptable en LL/BJCC. En realidad el tema del SPS es mas a tu comodidad con BLM.
It's basically an increase in damage. You don't lose anything by weaving transpose with a F3P since its instant. Basically, a one stack Astral Fire F3P is way stronger than F3P in Astral Ice. You should always maximize this type of transpose when you have a F3P during ice. If you do this for a couple of times during a fight, it's actually a generous increse in damage. Just be aware of your MP ticks.
Merciless is kinda hard at the beginning (the Bicorn sub-boss is legitimately challenging) but overall the whole game is trivialized when you realize you can pretty much blast through everything with proper personas of each element and high SP.