p.VDB, excellent questions. Sorry it took me a bit to respond but I could only make some crude measurements at home and wanted to check drawings and physical parts.
The answers concerning brake track width are a bit embarrassing. First of all, the 404 number on the web site is actually for the previous-generation 404. Second, the other numbers have been grabbed from the wrong columns in the spreadsheet that was used for the site and our catalog.
Here's the correct numbers for the top and bottom of the brake track (listed as top/bottom)
303 CC: 25.14/27.57
303 Tubular: 25.14/27.57
404 CC: 23.48/25.73
404 Tubular: 23.96/26.34
808 CC: 23.48/25.73
808 Tubular: 25.95/26.24
As you can see, these dimensions are identical for the clincher and tubular versions of the 303, relatively close on the 404, and there's a significant difference on the brake track top measurement comparing clincher and tubular 808. This is what was arrived at through experimentation with the transition from/to tire and rim at each of the depths and tire types.
In terms of the question of whether the clincher or tubular is faster there's no absolute answer. This will depend somewhat significantly on what tire models are being used on each version of the rim, as well as the effective yaw angle in question. Rolling resistance on various tube/tire combinations will also affect these comparisons.
The 404 and 808 (either clincher or tubular) are fastest (aerodynamically speaking) with 21mm tires, though design centered on 23mm tires. The 303 (either version) is still fastest with the narrow tires but design focused on minimizing aero penalties with tires up to 27/28mm wide. Again, like most things in aerodynamics, this isn't a hard and fast rule as it's still tire model-dependent. One company's 23mm tire performs significantly better on each of these wheels than their 22mm tire, for example.
Lastly, I threw together a quick plot of 23mm and 23mm data for 303 and 404 carbon clinchers. Sorry it's not super polished but again it's from our internal database.
Screen Shot 2011-12-12 at 10.03.36 PM.png [ 61.77 KiB | Viewed 4189 times ]
Hope this helps, and sorry for the confusion on the brake track widths.