Guten tag!
This is an excellent site about automotive lighting. If you are still in Deutschland, you might have e-code housings, in which case they throw the light on the road better than the DOT standard used in North America. If that is your case, you could use overwatt bulbs as long as you used relays and didn't go too far (You can melt the connector or housing if you get too crazy on the wattage)
This describes the idea behind using relays to supply power to the bulbs:
Daniel Stern Lighting Consultancy and Supply
One other thing, You can use osram silverstar bulbs in Europe. They are not the same as the Silvania Silverstar we have here, they are better. They do not tint the lenses, so you get more light on the road.
You can buy pre-made relay kits on ebay and other places, but if you don't mind doing some soldering/splicing you can do it yourself a little bit cheaper and customize it for your vehicle.
