Aero

The short answer to your question is that, generally speaking, going 10mm lower at the front will improve your aero performance. What is not certain is that your power output may suffer due to being more bent (this becomes more and more limiting the sharper your joint angles get). Your question is studied in depth by the pro teams and is the main reason for putting someone in a wind tunnel...  to find their most aero position while still putting out optimim power. All pros can get more aero, but their position is the best balance between drag and power delivery.

If you just look head on at anyone on a bike - the rider is a far bigger area to the wind than the bike.

So your position will have far more impact than the aero tweeks on a frame.

Lowest is not always best (i.e. most aerodynamic) and there's a point beyond which the aero gain is outweighed by loss of power. I've talked with & read of TTers who have found this for themselves.

It seems that, for road bikes, riding on the hoods with arms bent is better than riding in the drops. As with anything related to body position, being able to sustain it for the duration of your effort is more important than anything else.

But I'm sure there are plenty of people with data. Most of them, including Aerocoach, will charge for the privilege of sharing such information, as it's how they make a living. Xavier Disley occasional puts interesting numbers / charts on twitter. It's important to know what you're looking at and how the comparisons are done.

Manufacturers don't really want you to know exactly how their frames compare. Specialized's Win Tunnel team (Youtube playlist) have some interesting data but of course they are working for a single brand. Tour magazine compared a bunch of aero & non-aero frames and the resulting chart was tweeted by Cervelo. The difference between Giant's Propel and TCR models was 0.41% - that's 15 seconds per hour. Venge v Tarmac and Foil v Addict were very similar.

GCN have made some 'real world' videos, though often the results are inconclusive and I'd suggest that they are inclined to put entertainment before data (that's an observation, not a criticism). CW have also done a few videos and articles, including Michael Hutchinson circulating at Herne Hill in the name of science and the aero 'bang for buck':

http://www.cyclingweekly.co.uk/videos/watch/aero-kit-gives-bang-buck

http://www.cyclingweekly.co.uk/news/latest-news/time-trial-helmet-vs-roa...

http://www.cyclingweekly.co.uk/videos/cycling-tech/much-position-affect-...

An article by Cyclist mag - http://www.cyclist.co.uk/in-depth/534/how-much-time-can-aero-kit-save

Wheel manufacturer Flo took an interesting line, giving calculated time savings using what they call Net Drag Reduction Value, based on wind measurements on Ironman courses. They post some fascinating stuff on their blog.

I forget the exact words he used, but Chris Boardman said something during the Tour along the lines that  skinsuits (including dimples, chevrons and other tweaks) were where most gains can be found.

Loads of stuff on this, particularly (in my experience) on tri forums, but there are so many variables to measure and compare, that if you have picked two, you might be the one to have to do the comparison.

As a starter, though, this 'study' finds that the position on the bike is the most important drag factor http://www.aeroweenie.com/assets/backup/cervelo_frames/cervelo_position....

and this data found that using an aero frame really wasn't that much of a factor https://cyclingtips.com/2010/04/biggest-bang-for-your-buck-in-time-trial...

That same data found a skinsuit to be more beneficial than aerobars, but then you have to work out what they're comparing against as a baseline and how it applies to you.

Most 'studies' are of a small sample set in very specific conditions (wind tunnels) so I take them with a pinch of salt: I have never raced in a wind tunnel.

There are some rules of thumb, and the main 'takeaway' in all of this for me, is that it's the non-aero lump that powers the bike that is the main drag factor by quite a way - so try to 'get more aero'. Most riders probably could on most bikes - however, putting the rider into an aero position is *the* massive advantage of a dedicated tri/TT bike (not the slippery frame or wheels).

And, as simonmb and 700c say, other factors then come into play, particularly when tinkering with a bike that you probably initially specced for another reason (a sportive, club run, commute etc). Can you simply slam your stem without changing your seat position? Does that impact your comfort or ability to put your watts out for your ideal time? Could you get more aero anyway by tucking your elbows in and tweaking your seat etc etc? By the time you're into this 'marginal gains' territory it becomes very personal.

I think you'd undo any benefits from the aero gear if your position wasn't sorted. Rider and bike have to be considered as a package and how they interact, so I think the premise of the question is slightly flawed.

Not that manufacturers are letting on! They just want to push the latest thing and that's the 'aero bike'. It was weight a couple of years ago.

Not to mention the fact that a 'GC' bike (if you know what I mean) will be stiffer and likely more comfortable so may outweigh any aero advantage of the 'aero bike' on certain courses.

Good question. You'll find it is something triathletes an timetriallists have studied and experimented with for years and although I'm sure the bike industry knows the truth they haven't shared it because there's no money in it for them.

It's impossible to give the specific quantitive assessment you're asking for because body shapes vary as does the way they react to positional changes, but clearly the body is the most un-aerodynamic component on a bicycle. We already know that getting in the drops and tucking our elbows in will see our speed jump for the same effort. 

However, an aero position is achieved a greater metabolic cost (it's not as easy to breathe when your diaphragm is folded over on itself) - it drains us of energy in the way that fitting deep rims never will - but the increase in efficiency garnered by the reduced drag makes it worthwhile.

Look at the best timetriallers and note their positions. This is the key to their success, and it's something that has been honed over years of experience. 

Skinsuits, stubby helmets, disc wheels etc are all part of the manufacturer-fuelled 'arms race' and provide unquestionable benefit when used as intended, but far geater gains can be achieved for more cyclists through honing their positions. 

Regardless, aero bikes with deep rims and tear-drop tubes look fast. And beautiful.