I did try some in a pre Horizons beta and it took 3 homing missiles to kill a Sidey (not sure what missile class though), from what I've seen in video's they're much better on ground targets, with the tiny imperial and fed fighters coming they might be really useful as they're real difficult to hit.
Check out VJ's ground assault with an FGS with dumbfires
https://www.youtube.com/watch?v=npRVZKyvcws
Did a few tests in my D rated Cobra 4 and three class 1 homing missiles will kill an unshielded Sidey (108 armour) or a Hauler (90 armour) an Adder with 162 armour is a lot tougher so each missile reduce the hull by 35 - 40 AU (armour units?) as different ships have differing base armour quality that's just be a rough guide.
Here's a few video captures
https://youtu.be/FOJfIDXtnBI
https://youtu.be/81fupV2U-pU