I thought about that, and though technically you wouldn't add weight, you are probably right that both cases have the same net effect regarding overall buoyancy. Maybe I can put some heavy plates underneath my feet in the fins and see what that does, and just lose half a kg or so on my belt, just to see how that fares.
However, as a note, ankle weights, just like actually heavy fins like the scubapro jet fins, make kicks a heavier exercise due to the added weight (i.e more effort). Whereas lightweight negatively buoyant fins keep the effort much lower.
On my next trip I'll borrow some fins locally to see how they compare.
As for the tank, it's pretty good where it is. my trim in general is fine. perfectly horizontal, and when swimming there is no issue with the fins, only with hoovering and manoeuvring when stationary.