50w bulbs may be too much for the early C4 light housings. The entire fog/turn/parking lamp housing is a single sealed component so there is more risk of getting moisture into the housing if you drill any sort of holes to allow heat to escape.
The other thing to consider is the size of the factory fog lamp wire. The wire gauge is designed to handle a certain current level and adding bulbs with higher current draw will cause the wires to get hotter than normal. The stock wireing may be able to handle the increased current but if there is excessive resistance in the wiring already (corrosion due to age or broken strands in the wire itself) adding a higher current draw may lead to excessive heat and a potential of melting the insulation or maybe a fire.
You may also experience blown fuses with higher current bulbs. While you could install a higer amperage fuse, the purpose of the fuse is to protect the wiring, not the lights.
You can try it to see if it works, but if a fuse blows or it appears that the housing is showing discoloration around the bulb, then go back to the original bulb.
FWIW, there is a 38w bulb that may work to give you better light See if Osram or Phillips has an 882 bulb or similar. Your parts place should have a catalog with a cross-reference listing that will show bulb types with different wattages.