I think it would make the game more fluid if we could exit the low prone stance by simply hitting any of the movement keys. It should be possible to move quickly or initiate a roll while in low prone.
Basically, pressing the movement key switches the character back to standard prone and allows the player to move right away. The character is already in the prone position - except lying down sideways, so I think this is a logical feature that should be implemented. The current system where you have to change stance by using the modifier key feels slow.