The standard replacement is 4S A123 to replace a 3S LiPoly.
The 1100 cells ahve roughly twice the IR as the 2300 cells. This is also true for LiPolys with same C ratings. That is a 2000 mAh 20C will have twice the IR of a 4000 mAh 20C all things else being equal.
My Foamies which I flew in the past on 3S 1320 TP ProLites fly fine on 4S 1100 VPX pack. More volts under load.more watts but 3 oz. more weight also.
In case you are unaware of my VPX thread here