I can tell you for sure that I thought I was bright with my 5 watt bulbs until I next looked at the back of the cluster. Definite melt around the bulb mounts. Since the back of the cluster is also the pc board for much of the instrumentation, you don't want it melting.

I went back to the 3 watt bulbs. Dimmer is brighter, in the long run. Perhaps I'll eventually try led's. I'm sure taking apart the cluster and cleaning it would also be beneficial, but what a pain....