Oh, what speed grade did you use for the V6? All my boards with 130s and 240s (ml605) are -1, so if you used -3 that could explain the big difference in the quality of the results.
my board have 2 pcs 130T devices. the speed grade is -2I. so your design with DSP48s really have some speed and power usage advantage. BTW, can your design be synthesized on XST?