I joined the board just to talk about the benefits of this thread. [Thank you to the contributors, btw].
I had a 53cm TT with a 13 HT/12cm stem with 42cm bars. Been riding this setup for 3 years.
I went to a 51.8 TT with a 10.5 HT/12cm stem with 40cm bars.
Wow. What a difference - it took me about a week to dial in. I try and ride as aggressively as I can. So this path of reducing drag coefficient may not be for everyone.
Couple points of feedback:
My saddle to crank length had to drop ~1/2 inch from first setup to accomodate the lower front end, because I found keeping my old saddle height created too much stretch for the hamstrings. It makes perfect sense though, if your drop dropped - you would suddenly find your quads ramming into your chest, the hamstring angle would be extreme, and difficulty getting blood to the legs because all your body parts near the saddle are now being squeezed by your upper body coming down. Saddle height and fore-aft are coupled, so I had to move the saddle back a little, down a little, front a little, etc - to get a really good position for my hips/quads. Not sure what the exact procedure is, but I found I spent a lot of time fiddling with these two positions until I got it just so. The result is, a good powerful stroke, more hip rotation from previous position, stretched flat back, and lots of room between my quads and lungs/rib cage [almost triathlon position like].
The benefit to dropping the front end - massive aero benefits. My bib straps form a nice horizontal ellipse in the mirror [while on the hoods!]. Very nice. Very comfy too, because I shortened the top tube. How comfy? When I'm in the drops, my shoulders are almost vertical and relax, allowing me to get even lower without tension or hold the position for an extended period of time.
I will never go back to 42cm bars. 40 is where it's at. Small chested/shouldered riders should really consider trying 40cm bars. I spent 8 years on 42's. Call me ignorant of change.
Power: I didn't really notice a big difference. It didn't drop and it's gone up maybe 10W for both 1hr and 5min, but that could just be from regular training, so it's hard to say.
The biggest difference for me is creating more space for the lungs and general comfort. The old setup was longer, higher, and wider - I was quite stretched out on the hoods. Maybe too stretched, because now, I feel like I am not straining to breathe during hard efforts [but it is hard to actually determine whether it was the effort or the position which creates discomfort]. I feel it is a very aggressive position, almost too much [?], because in any hand position - wrists on bars/holding bars/hoods/drops - my back is between 20 - 0 degrees to parallel. I haven't gotten a chance to do a 4 hour jaunt on this setup, so the long-term ride report will reveal itself in the spring.
Thanks again - we'll see what happens in 2013...