Bugger, Atapene got in an essay while I typed three lines!

Duck is correct and also incorrect. Any power that doesn't transfer at all (movement within the pocket for instance) is wasted. Where he is partially incorrect is that movement that is delayed (through soft footpockets, etc) is not lost completely. His suggestion that this reduces performances is definitely correct.

How's this for a concrete example. A freediver friend got a new monofin recently. The footpockets fitted, but his toes had some vertical and horizontal movement. Simply by adding extra hard rubber in under his toes and to the side dropped his strokes/length from 5 to 4 at the same pace and effort. That is significant.

There are definitely other major performance characteristics, but footpockets are often overlooked, despite their importance. Personally, I think you need better the improved fit of custom pockets so you can increase the stiffness of the footpockets themselves. Their flex criteria are more important.