I wouldn't bother with higher wattage bulbs, although LED's seem to be OK. The previous owner of my car installed some 5 watt bulbs and I have no idea how long they were in there for. Part of the bulb housing melted and it didn't look too pretty. The 1986-1989 W124 cars have smaller illumination channels for the bulbs, and these are improved in the 1990 onward bezels.

Even with the stock Euro wattage bulbs (2 or 3 watts) the lighting was much better compared to when the higher wattage bulbs were in the old cluster!