Your assessment of the panhard bar situation is correct. The oem wheel/tire fitment left a lot of room for mis-alignment. Your new larger wheel/tire fitment requires closer attention to centering the rear axle to avoid interference.
In addition, when you lower the car the resulting panhard bar angle change tends to pull the body toward the drivers side, lessening the tire/fender clearance on the passenger side. In addition to that, the resulting angle change of the lower control arms will tend to pull the rear axle forward, resulting in the fender interference that you have described.

The fix? [here comes the shameless self promotional part]

http://www.ridetech.com/products/Che...s-917-923.html

The panhard bar is adjustable and the control arm lengths have been optimized for you new lowered stance.