a big honkin' mudflap off the front fender, big shoes with wool socks
a good head/wind/rain covering so as to not be distracted by drip and spray.
the distraction of feeling like a soaked cat in a monsoon can knock your speed down a couple of K. You know, when you're riding, the water is streaming down thru your helmet vents into your face with sweat and hair product stinging your eyes, fog in the glasses kind of riding....
Also, the biggest tires you can stuff between the stays.
for winter rubber, bigger IS better. I recommend going 'cross' if your bike won't take at LEAST a 700x32c with fenders for the best traction for rainy riding.
but its true, you just can't or shouldn't go as fast. how about these evening Seattle commutes this last week, eh people? I can't even imagine drivers seeing me very well on Thursday night, thas' for sure!