Even if the leverage ratios are a bit different, in practice they completely interchange. I have Campy Ergo brifters on two bikes with Shimano DP brakes, one set of Dura Ace and one set of Ultegras. I also have a couple of bikes with Shimano STI brifters and Ultegra DP brakes. The "feel", modulation and braking power are identical with both set-ups.

BTW, all of my brakes have Kool Stop salmon pads so they are identical in that regard. If someone reports a noticable difference when they mix-and-match brake and lever manufacturers, they may be seeing differences in brake pad composition.