I've changed from STi Brembos to 5570 330mm AP 6 pots for track use (now using PF discs and RS29 pads). Despite a slightly smaller total piston area they stop better and are more consistent. Best of all is the smoothness and progression and clean release that is just a pleasure to use. Call me sad but I actually enjoy braking as much as having a blast.
As Alyn on here told me, the truth is that the Brembo is a budget caliper and you get what you pay for.
I changed to the AP 355 conversion and tried the DS 2500 and DS 3000 pads, Both gave some problems and we went for Padgets, which I'm still using. Lots of noise though when they aren't really hot. On track we found that we needed to add some ducting due to temp but now we are back down to 600deg they are awsome. Very good feel, and very progressive.
If you have the money then get them, they are so worth it.
As JB1 says (and I forgot to mention) ventilation makes a huge difference. I don't have ducting but I have cut large holes in the insides of my front wheelarches and leave the fog light covers off (Blob STi). Gets air behind the wheels and does the trick. Also put similar pads to rear brakes to keep braking effort even and not overload the fronts.
OE Brembo discs weigh 10kg, Discs on bells PF for Brembo or AP are both about 8kg so either a useful saving. Also decent lightweight wheels PFF7 ot TD 1.2 are 8kg vs OE STi golds at 12kg. Difference is noticeable.
Strip interior of an STi and fit lightweight seats saves c70kg, sadly roll cage adds 28kg but stiffens chassis.