When this post appeared I was on vacation, so here's my take on it.
First, my qualifications: I have done dozens of brake conversions, in fact it used to be part of my job description, and in that time I learned a thing or two hundred about the ins and outs of hydraulic braking systems.
Let me dispel any misconceptions about "better" master cylinders: a trick radial master that worked wonders on a ZX10 (or whatever) would probably be a disaster on the 919 for the simple reason that it would almost certainly be too large for a proper hydraulic ratio. Make no mistake -- when sitting in the garage it would feel very
impressive, with about 1/4" of travel at the lever end before feeling like it ran into a steel bar. On the road, however, your opinion would do a 180 degree turn the first time you tried to stop. Hydraulic brake systems are all about leverage ratios, in this case the ratio of the area of the master cylinder piston to the areas of all the pistons in the calipers times the leverage ratio of an average position of the index finger on the lever to the length of the lever cam: arbitrarily 3:1. The 919's ratio is 33.42:1 (14mm master, four 30.15mm and four 27mm caliper pistons) times 3 . These ratios combined with the Cf of the brake pads translates a 10 Lb pull at the lever to 1002 Lbs at the pads, and has you squealing the front tire. If a "trick" $500 master cylinder with a piston of 18mm is fitted the ratio lowers to 20.2:1 * 3, the same 10Lb pull becomes a paltry 606 Lb shove from the calipers, and wide eyes as you aren't stopping nearly as fast as you used to. In fact, it would take 60% more force at the lever to equal the stop of the original master.
There are things you can do to decrease the initial travel of the lever, and none of them involve replacing the master cylinder. First and foremost bleed the brakes thoroughly. Second, convert to full floating rotors to insure the pads are never knocked back by the inevitable runout of even semi floating rotors. Third, adjust the position of the master cylinder piston with ring shims to bring the primary seal right to the leading edge of the compensating port, minimizing free travel. Fourth, make sure there is no free play between the cam end of the lever and the end of the piston. Fifth, and by far the most difficult, is to work over all the caliper pistons with crocus cloth to where they all start to move at the same time, eliminating sequential engagement. I have done all of these, and it definitely makes for a very
positive lever, but to do it properly takes the better part of a week and lots of knowledge and experience.
Frankly, I'd prefer to ride for that week, and live with brakes that will howl the tire with one finger! Which, incidentally, is what you will end up with after that week of extremely picky work: it just starts a couple milliseconds sooner. Unless you're really anal, it's not worth it.
I have a 929 also. The 929 has very good brakes also. SS lines there also. But they are not a whole lot better than the 919. The only thing I question Rob about in his thread is when he assumes the 929 uses soft pads. My first set went nearly 20,000 miles. I'm not saying he's wrong, just questioning....
In this case, "soft" means "20% more pad area and less pressure needed to obtain equal friction", not necessarily "wears faster". Just a difference in viewpoint.