SDK vs SFSK

Does anyone know if submerged dolphin kicks are always better than submerged freestyle (or backstroke) kicks? I find fly kicks very tiring and slow for my particular body. It seems I can go as fast if not faster off the walls kicking free or back underwater--and these take much less energy. If SDK is the obviously preferred approach, can anybody provide actual evidence--swimming science studies, for instance--that compare the same person's speed doing both? Similarly, is there some physical cause for why split-leg kicking should be intrinsically slower than legs-together kicking? Obviously, you have to do SDK on fly and *** pullouts (if you are going to do any kick at all.) But for free and back, you still have a choice, and I'd like to know if the SDK choice is always (or almost always) a better one. I am wondering if it is sufficient to conclude that because most of the world's fastest swimmers do this, it necessarily means it's an optimal technique. Dara Torres, for her part, does not use SDK in her sprints. Is this just a case of old dogs having trouble with new tricks? Or could it be that SDKs work great for some--but not so great for other body types?