I made a booster brace out of a spare arch from another centerpull for these brakes too. It's hard to see them in the photos I posted above because the bag was in the way, but when I was experimenting with the brakes on another bike to see if I liked them I took a few photos.

Pre-polishing photos so the brakes don't look like much at this point, but you can at least see the brace:




The brace made a whole lot of difference in how much the back arch flexed and twists as the pads set down on the rim. It made the difference of the lever hitting the bars solidly with medium-high effort to then almost taking two hands to get the lever to barely tap the bar tape. The pads visibly squish a LOT at that point. I also have a very strong modern fat alloy cable stop at the top of the headset (10mm stack height!) -not the flexy thin steel ones from vintage bikes. So there is very little flex anywhere but in the brake arms themselves. The arch can't flex outwards when the arms press inwards.

I didn't bother putting a booster arch in the back. The rear brakes don't need that kind of power. The fronts have awesome stopping power, as much as this bike had when I was using Ultegra dual-pivot calipers. The return springs on the Dia Comp are much stiffer than those on the modern DP caliper though so it takes a bit more hand strength to get the same amount of braking power because of overpowering the spring. I figure in the old days they needed much more return spring due to the inferior cable housings back then. Modern brakes can be built assuming that lined cable housings don't hang up as much so the brakes don't need as much spring-back to pull themselves back open and slide the cable back thorough the housing.

Perhaps due to the higher lever pull there is slightly more feel at maximum braking? When the lever is let off the brake backs off quicker?

It's hard to tell. These brakes are pretty awesome, and because of the booster I don't need excessive toe-in to keep them from squawking and making noise under hard braking. The only time they make a peep is if I am braking really hard for that last 12" before the stop they make the slightest of herp of a noise as they come to a stop. They work very well in the wet too as I was able to get some good testing riding in a rainstorm last week for 50 miles. The salmon KoolStops made a mess of the rims and the gumwall tires in the rain, but they do that...