From the research I have been doing on the various Nissin calipers fitted to Hondas, the spacing of the mounts and their relative position to an arbitrary datum (the midpoint of a line drawn from the center of the trailing piston to the center of the leading piston) is the same to the limits of my PME. This implies the mounts on the fork legs are positioned to properly place the calipers on whatever diameter rotors are fitted. From this, it also folllows that if you are fitting RC51 calipers to the 919 they will bolt right on and place the pads close to perfectly on the smaller rotors. At least the 929 calipers I got from EBAY dropped right onto the 919 forks and the pads were right where they needed to be, made more significant by the fact that the 929 has 330mm rotors.
There is a problem with the dimensions posted by mcromo44 -- checking with dimensions taken directly from 919 & 929 pads that I have here the width is right but the height is way off. I can only conclude the dimension they give is from the center of the retention hole to the lower edge of the pad, not the total friction material height. Since the width of the friction surface of the 919 rotor is 32.3mm and the height of the pad is 32mm, it would seem this is a more accurate way to do things. BTW, the height of a 929 pad is also 32mm. See the first illustration for a comparison.
As you can see from the revised dimensions the 929 pads have roughly 9% greater area per pad, a total of 36% larger friction area total.
Something else I found recently is the radial calipers fitted to a 2009 CBR1000rr has exactly the same piston diameters as the '94 CBR 900 calipers I had a while back: 32 / 30. A bit surprising.
Let's look at a breakdown of master cylinders, radial and otherwise. Here's an article on the Brembo radials:
Overview of a Brembo Master Cylinders
As you can see the lever is quite straightforward -- essentially a straight run from your fingers to the piston. Cleaner actuation? Not by virtue of what I've said so far. Let's take a look at both types and work it out.
First, a conventional master:
The brake lever pivots at the master cylinder body which transfers motion through a right angle to the actuation point at the piston. The average mechanical ratio is about 3 to 3.5 to 1. The disadvantage to this setup is the angle from the centerline of the piston to the mean center of the lever pivot changes substantially from it's at rest position to the point where the brakes apply, an average of 4 to 5.5 degrees. In order to accomodate this change the actuation arm scans across the face if the piston, creating a thrust load on the piston until the angle is 90 degrees, and ideal point where the brakes should apply. The end result of this is the piston binds very slightly in its bore, using up a small percentage of the force you are applying. It gets considerably worse when the lubrication between the arm and the end of the piston breaks down, where the binding force gets high enough to practically freeze up the lever action and wear the bore and seals badly. Not one little bit of good!
On to the Brembo 19RCS:
The radial master is different in that it uses a larger piston size, in this case 19mm, and changes the mechanical ratio by a couple of methods. First, the actuation arm length is 18 or 20mm, about 38% to 25% shorter than an average conventional master. Additionally, the distance from the pivot to the point where your fingers contact the lever is greater. Why? The pivot on the conventional is forward of the handlebar mount by an average of 19mm, while the radial pivot is behind the mount by roughly 15mm, making a total difference of 34mm. Add that to the length of the conventional lever and you have a mechanical ratio of between 6:1 and 5.4:1. Another advantage is the angle change is considerably less -- about 1.5 degrees versus 4.5 degrees. This results in less binding and a smoother feel. Of course the fact that the Brembo is a high end unit means more attention is being paid to surface finish of the bore, teflon lipped seals may be used, and the lever and pushrod pivots are of a higher quality. The result is a smoother feel under pressure and cleaner actuation. This is what retailers are talking about when they say it works better, but they invariably attribute it to the position of the lever and piston. The truth is I can and have built conventional master cylinders that were every bit as smooth as a radial by adopting the same methods Brembo uses, but it wouldn't sell simply because it isn't a radial!
Let's break it down. Take a system with two calipers with 4 pistons of 32/30 mm each and a conventional master of 17mm. The hydraulic ratio is 26.63:1, and when multiplied by the mechanical ratio becomes 93.2:1. Fine and dandy. The 19mm radial master has a hyd. ratio of 21.32:1, and multiplying by the mechanical ratio gives a total ratio of 127.9:1 to 115.1:1. The end reslut is the brakes will be more sensitive, but due to the better quality components and attention to detail the feel should still be good.