I'll add my $0.02: Simple answer is the spinnaker has the most effect. By far.

I think there are a couple reasons:
1) Sail size: 21m^2 vs 4.2m^2.
2) Sail cut: the jib is cut very flat for tight upwind wind angles, while the spinnaker is deeper cut keeping attached flow at higher wind angles that are seen heading downwind.

So on the typical downwind run, I think the jib is mostly just enjoying the ride and has a very limited effect on the main's lee-side flow. That being said, every little bit helps, so the jib should be trimmed out with lots of camber. In my case, the top of the leach is almost completely open and not doing anything, but the mid and lower sections appear to draw a little bit - judge by telltales.