I had always assumed that weapon strength was what determined your invasion power for taking planets. But thinking about it some more, since defense adds to MP perhaps defense also adds to invasion power. Does anyone know off hand?

It would be very nice if there were any way at all to view ship/fleet invasion strength. I cannot believe this is still missing. AFAIK -- but there is no way to prove it -- the invasion strength of a fleet is (normal MP (including armor and defense) + invasion only MP due to invasion modules) * invasion scale factor from plasma payloads. Notice that plasma payloads give +30% invasion strength, so putting say 10 of them on a DN "should" give triple the invasion strength.