Mike,
Like most things in autox, I think it depends on what your true goals are and how you prioritize them. I now rank ease of modulation in autox well above absolute performance.
I put a set of HP+ on the Vette for a season. They dust very heavily, and they do squeal. Annoying, but that's not why I took them off after a season. Here's why:
The initial bite is tremendous. For a car without ABS, it might be too much so, and I can easily see how it would result in lockups on non ABS cars, especially on gritty surfaces. When you first put them on, you really think "oh my gosh, these things are awesome!"
The reason I got rid of them is because they made a driving habit I have even worse at the time. They stop the car so well that they work almost too well for autocross, for my driving preferences. A slight amount of pressure has a huge effect. It seemed like I could just touch the pedal and the nose would dive.
On the good side, I did not find them to be temperature sensitive at all. They stopped my 3400 lb car just fine at the end of first straight at an autox.
They were not easy to modulate to get the exact amount of braking I wanted. I'm trying to get away from overslowing the car and I do a whole lot less threshold braking than I once did. Jim Pastorius was actually the first guy I remember who wisely kept beating on me about braking sooner than I was and overbraking. (thanks Jim!)
And for me, I found that since I was over-slowing the car, I needed to brake a litttle less. With the HP+, I only had to use relatively slight pedal pressure to practically threshold brake the car. My leg and foot and instincts just didn't adjust very well to trying to slow down less with those pads.
For lack of better direction, I then went with the Performance Friction Z rated AutoZone pads. They do fine but they don't feel as "dramatic". Braking force seems perfectly proportional to pedal force to me, which is what I really needed. In fact, when I first put the PFZs on the car after the Hawks, I thought I had a serious brake problem. I went to try and stop the car and I thought I was going to have to bend the pedal to get the car to stop. I went back and reinspected my brake job!
My foot adjusted quickly though and now it seems fine. The PFZs are ok, but I have no doubt that the HP+ may stop the car a few feet shorter than the PFZs.
I've learned that I have to find a pad that fits my driving style and my car's temperament, not just the pad with the best performance characteristics on paper. More talented drivers than I could probably make the HP+ work fine for autox.
I continue to hear things about the HPS that make me want to try a set when the PFZs wear out, in about five seasons.

There is something to be said though for finding a pad you like and sticking with it.
Hope this helps.
Miles