Once the Wing ANti Ice has been used, or the TAI is being used, the stick shaker speed is raised, and the min manouevre speed is raised, but the FMC ref speeds are not.
You've partially answered your own question there. With a Flap 15 approach the margin between minimum manouevre speed and Vref is quite small, and is made even smaller by use of TAI or WAI, thus the addition of 10kts to the Vref. With a F30 or F40 approach, the margin is sufficiently large not to require any additional 'buffer'.
The go-around manouevre margin is another thing, and is taken care of by the following instruction (cut from the QRH):
Maintain VREF 15 + 5 knots or VREF ICE + 5 knots minimum on final approach to assure sufficient maneuver margin and speed for go-around.