5w is the legal maximum for side light bulbs. So to remain legal the only alternative is led bulbs.
The downside is that they are usually much 'bluer' white and make the headlamp bulbs appear yellow!
The 501 type bulb is available in higher wattage, I've seen them at 11w but as I said, not legal.
The higher wattage 501s are used in 12v garden lighting.