Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
My 500 has been with me since 1980 and always had the 55/60 Watts halogen. The Philips X-treme vision or Motovision is stunningly good and leaves nothing to be desired. If you're really worried about charging (you shouldn't) you could choose the Philips Ecovision that has the performance of a very good 55/60 Watts halogen but draws 20% less. Philips always came out best in tests. Recommended.
What's surprising though is that if the later 550s came with H4 bulbs and these would normally be 55/60w bulbs why wasn't the alternator uprated to accommodate the extra wattage.
The problem is that with electronic ignition and the headlight on my battery is slowly discharging unless I'm tearing along, hence I need to know if HID is effective as a means of lighting the road ahead.
I thought that the main issue with HIDs is that you only get the correct blend of power and beam pattern with OE installations, because the reflector shape is designed to work with HIDs. Putting HIDs into a reflector designed for conventional bulbs means that you get the brightness, but the beam pattern will probably be wrong.