i don't have the specifications for the v6 rotors (whether they're thicker/bigger). if they are bigger/thicker, chances are you'll have to also get the v6 brake calipers to make everything work.

there's a couple of possibilities going on here in your scenario:

1) you're pushing the brakes to the limit but you're not allowing it to cool off properly, thus you rotors become glazed by pad deposit.

2) you're pushing the brakes BEYOND its limit thus experience fade.

if you're having fade, then it may be wise to invest in a bigger rotor kit (not so much because of x-drilling or slotting, but because bigger rotors are more resistant to temperature change.

if you're just warping rotors, then you're not allowing the rotors to cool properly. for instance, if you brake from 100-0, and keep the foot on the brakes (especially because if the car is an automatic), then the rotors are being cooled unevening when the car isn't moving. the part of the rotor touching the brake pads will cool at a different rate than the rest of the rotors exposed to just air. you end up glazing the rotor and leaving imprint/deposits of the pad.

the cure to this obviously is having brake setups like Porsches and Ferraris (whose rotors are huge, directionally vaned for cooling), designed for high speed stops.