Just ran a planet assault myself. After deliberately breaking the GW3 rules by taking a CVA+ and a DVL (in my defence, there was nobody else on at the time) and equipping them to the nines, the final AI draw looked something like:
Attackers: F-CVA+, F-DVL, H-PAL+, H-SAL+, plus about half a dozen Orion FF-CL weight vessels.
Defenders: L-Planet (apparently unarmed), 2 K-SBs, 2 L-FRDs, K-D6B, K-D5C, K-C8B, K-C9B and an L-POL that turned up 2/3 of the way through.
Result? A 20-minute slugfest, most of the fighters dead, most of the Orions vaped, a lot of dead Klinks and Lyrans and about 2800 prestige.
Conclusion would seem to be that multiple DN-weight vessels are needed to make an assault on a belligerent planet stick (which is as it should be)...though that is just a bit excessive for a neutral backwater.