While the placebo effect is real and shouldn't be ignored, it seems like the consensus at this point is that any actual energy losses to frame flex (within reason) are minimal to nonexistent. Modern frames within a given category are pretty tightly grouped when compared to bikes from ten, twenty, or thirty years ago. Some people prefer something that feels stiff, others like a lively ride- and neither is necessarily right or wrong. As much as we like to think of ourselves as rational, for a lot of us bikes are emotional purchases.