McDyess over Bonner is an improvement in the interior -- in terms of post defense, rebounds, blocked shots and overall toughness -- but Bonner is the better perimeter defender. McDyess for Bonner swap would probably improve the defense, but only slightly.
Where the improvement can come next year is Jefferson replacing Finley. If Jefferson can revert back to his defensive roots, that's a gargantuan improvement over Finley. That combined with McDyess helping protect the rim is where the improvement begins.
Then if Hill can carve out some time off the bench and Mason can show some signs of improvement, the D can be a lot better.