There will be a small but real aerodynamic benefit from going with narrower tires as well.
See this BMW data:
http://ecomodder.com/forum/showthrea...g-cd-7475.html
Will 155's fit properly on the rims (width)?
In "the realm of safety", as you put it, I expect you'll likely see reduced lateral grip in the dry (not certain about wet - depends on many other factors). Whether you're OK with that is up to you.