OK, I tested it out and immediately ran into some issues. With the same settings as I always use, I get results that are somewhat counter-intuitive.
On the campaign map, I load up the game and all is well. Then I try and pan the camera. It *inches* over to the right, my FPS hovering around 8-10 (normal is 40-60.) OK... Well, it's annoying, and it takes *forever* to rotate, pan, or zoom the camera to be able to select a position to go to.
I entered a battle, with just me and 4 river pirates. It loads up, and the battle scene is actually much more stable than the map. I'm getting anywhere from 12-24 FPS on a grassy plain with few trees and hills (normal is 20-30) which is about what I get in a forest scene (this is why I stay away from forests and get annoyed at mods that are very liberal with forests :p) At this point, I can conjecture that in a forest scene my FPS would likely drop below 10 consistently.
My M&B settings:
Character shadows ON, Infinite ragdolls/corpses, all other in-game options OFF
Bump Mapping ON
Fast Water Reflections ON (I actually have no idea what these last two do)
Texture Detail @ 50
Anti-Aliasing OFF
Resolution 1280x800
Shadow Quality LOW
Render Method: DirectX 7
My Computer's specs:
Processor: Intel 1.4 GHz
GFX Card: GeForce FX 5200 32MB
RAM: 512 MB
If you need more info, do tell.