Video explains CoG effects.

[MaxGunz]

With different fuel loading the P-51 is supposed to shift since a patch a while back now.

I can't remember if dropping bombs makes a difference. Most bombers are loaded at or near center, jabos are, so you wouldn't notice.

Added: what I get from that is that the positive control stability that makes a good gunnery/bombing platform costs drag in proportion to the stability.
Nose-heavy takes a heavier push down on the tail and that induces more drag.