Heres my 2c coming from actual driving experiences
I had standard calipers with DBA 4000 rotors and good pads, I had no where near the braking control I do now, I fact it was woeful and pinacle braking was really hard to pick...now it is easy to do with out thinking. Further more they could over heat after a decent drive in the mountains let alone at a track day.
And the Brembos I got were half the price of a Grex or similar kit...in fact they were cheaper than G4s!