Tire wear/toein/toe-out

Back when most cars had positive camber (front wheels leaned out at the top), we heard that to avoid excessive tread wear those wheels needed toe-in to compensate for any wheel's tendency to curve in the direction it leans. Now we have severe negative camber (Prius rear wheels lean in 1.5 degrees, +/- 0.5). This would seem to call for toe-out to compensate for the wheels' tendency to roll in a curve toward the center of the car. To the contrary, Prius specs call for slight toe-in. Could this be a factor in the excessive tire wear reported by many Prius owners? Has anyone out there experimented with shimming Prius rear wheels to toe out instead of in? If so, how much toe-out is or would be appropriate?