For what it's worth here's my K/D rate against each of the three heavies over the past 6 months as a guage. These have come mostly in, in order, the Spit XVI, FW190A-8 and BF109K-4 from every concievable attack angle (of these the Spit is the most dangerous to attack with, the A-8 the least):
B-17 - 1.78
B-24 - 3.90
Lanc - 3.62