I think our only options are the wavetrac and phantom grip. I am not sure about ratios on either though. I bunch of the SRT6s are running Wavetrac, maybe they would know. I know at least one forum member had a phantom grip, but I can't remember who.

Wavetrac
WavetracŪ Differentials - Application List

Phantom grip
http://www.ebay.com/itm/2005-2008-CHRYSLER-CROSSFIRE-SRT6-LIMITED-SLIP-DIFFERENTIAL-LSD-/200681577658?pt=Motors_Car_Truck_Parts_Accessories&fits=Model%3ACrossfire&hash=item2eb98ddcba&vxp=mtr