Here's the bottom line: HAWK HP+ PADS SUCK DONKEY DICK, and nobody who who's ever really used them hard on a Mustang's brakes will tell you otherwise. The brake upgrade people love them though, because everyone who uses them says "man, I have these fancy-pants pads, and my brakes suck, I must need a better brake system" And the brake upgrade people say "sure thing, step right over here and shell out $2500+ for a bigger brake system".
But 98% of the time, the rotors they already have are more than they'll ever need, they just need a pad that doesn't suck. If I had $10 for every person I've seen spend $2500 on a brake problem they could have cured with $200 worth of better pads, I could have retired rich by now. Slap a set of Raybestos ST47's on there and prepare to pray to a new god.
How does the fact that she's used the same pad on PBR calipers before with no issues factor into this?
Granted, the HP+ is labelled as an auto-x pad, but I still don't see how they worked with PBR's and not with Brembo calipers. Ana, did something else in your setup change when you went to the Brembos? Wheels/tires/etc?