You shouldn't have much of a difference in temp going from a 38w to a 50w bulb. As long as you are moving when they are on, the passing air will help cool them. I have almost doubled the wattage (100w) of my high beam lamps, and have yet to have any problems.
The only other thing you could do would be to give the 38w bulb a better power supply. I know from personal experience that rewiring the headlamps with heavier wire doesn't do anything on a Contour. Ford pretty much already did it by placing the relays right next to the battery. Find out how long the wiring is for the fog lamps, and where they get their power from. If they get it from inside the car, they will lose some power due to the resistance of the wiring. You could place a 20A breaker and a heavy relay closer to the bulbs. This will allow them to get the power that they need. I did this on my old Grand Am, and on the Ramcharger and have had excelent results.
