I mainly use the 12% position (known as the Nibali I think) but my problem is neck-ache from not doing it enough and also, I found that having your jaw on the stem put you at risk of any road bumps pushing the stem straight into your jaw. For the top tube one, I found it a problem getting safely back on the saddle with wearing shorts - not a problem with bibs though. however, I didn't like all the weight being so far forward if there was a sudden need to break. Like I mentioned, I've been trying the Pantani recently, and found it easy to use - being small, I don't have to put my btm far back off the saddle to have my stomach flat on the saddle and my btm is at no risk of catching the rear wheel. It takes a while to get used to the steering with the weight being in a different place, but braking was no problem, actually a bit easier with the weight being moved back. I might try it for my triathlon but as they DQ you for riding in an aero TT position, I'm not sure how they would take to descending in a funky manner
One of the comments about the testing has been the fact that all the tests have been done with the hands in the drops. Like you say, hands tucked in would seem to make it even faster.