First things first, you must find your User.ini file. It is usually located in C:\Users\[username]\AppData\Roaming\Bioshock2Steam. Your mileage may vary depending on your OS but it is somewhere in your user files and should not be -too- hard to find.

Next, open dat ♥♥♥♥ up with Notepad and replace (ctrl + H in Notepad) all

• w=moveforward

with

W=MoveForward | set Engine.PlayerInput MouseAccelThreshold 0.0 | set Engine.PlayerInput MouseSmoothingMode 0
The replaced text should be all one line!

Save your edited User.ini and for good measure set the file to 'read only' via file properties.
Now every time you load into the game and ress "w", your mouse will behave correctly!
Seeing as "w" is a key you should be good friends with, it's a small price to pay.
Enjoy Bioshock mouse acceleration and lag free!

Lazyass devs not fixing this awesome game's consolitis