540 calipers and M5 calipers have about the same torque, but the M5 has a larger fluid reservoir and more metal, allowing them to absorb more heat. if you want and upgrade, get stoptech steel hose lines and cryo treated slotted rotors.
i am also using the brass bushings, which cause the slot vibrations to be amplified into the suspension (kind of annoying when cold) but they do give it more consistent feeling bite. had to do a power stop 65-0 short notice the other day, was able to not lock up the abs but land 2ft before my "predicted" spot. any other brake setup and i would have had to slam the pedal all the way.
currently using 540 calipers with ceramic pads.
Alusil, Dinan DME | ITG Air Filter | Eibach/Bilstein HD | 26/20 Swaybars | Iridium plugs | Depo/ProLumen HID | Optima batt. | no AC | Stoptech brake lines, Frozen rotors, brass bushings, Superblue