Generally speaking, going from an Intel C2D / C2Q to Haswell and assuming the same clockspeed, the performance increase would be over 33%. Average performance increase is as follows:
C2D / C2Q ---> 1st Gen Core i3/i5/i7 = 10%
1st Gen Core i3/i5/i7 ---> 2nd Gen Core i3/i5/i7 = 12%
2nd Gen Core i3/i5/i7 ---> 3rd Gen Core i3/i5/i7 = 6%
3rd Gen Core i3/i5/i7 ---> 4th Gen Core i3/i5/i7 = 4%
Overall average performance increase = (1.1 * 1.12 * 1.06 * 1.04) -1 = 1.358 -1 = .358 = 35.8%