This has made playing the game very frustrating (bar the low fps on good systems)

When playing on multi-monitor systems(and only using one) the cursor will be visibly moving around between monitors as your character looks around.

This can be problematic when the cursor is needed(inventory management, ect.) and you can't find the cursor. In more serious cases, if the mouse is clicked while on another monitor it will dump you out of the game and to the desktop(or whatever application the cursor was over).

Problem exists in both fullscreen and windowed modes. Is also an issue in ArmA 2. The mouse cursor needs to locked and centered to the monitor the game is on, so when I go to the inventory the cursor is always in the middle, not someplace random on either monitor

This probably isn't an issue with a surround(game across all monitors) setup, due to the cursor having nowhere else to go.