Doffy's power really depends on if Bird Cage is a Move that constantly drains energy, or only takes one usage of energy.
If Doflamingo only needed to use one burst of energy to make the Bird Cage, he's not that strong, and the technique itself is busted. If Doflamingo needed to use energy to maintain the Bird Cage the entire time it was up, and was fighting while draining energy to keep the Bird Cage up, Doflamingo actually has a claim to be on a high Yonkou Commander Level, because he would have been fighting Law and Luffy after casting it.
If Doflamingo was fighting Gear 4 Luffy WHILE using energy to maintain the Bird Cage, Doflamingo is much more powerful than he's been given credit for.
Would love to find something proving whether or not Doffy had to use energy to maintain the Bird Cage.