Brabus didn't offer a 3.6L but they did offer a 3.0L turbo. I'm pretty sure they would have tested a 3.6L biturbo but the fact they didn't offer it suggests that it's mre trouble than is worth considering, only my 2 cents though.
As for the ECU it has been modified by Brabus by means of a piggy-back module which takes over at full throttle I believe. they ignition selector code (not sure if you have this in the US) has been changed to N for advanced timing, more power