I've gone round and round with headlights, mostly on my '83 R100. I like PIAA bulbs, but actual rigorous testing proved that the only way you get more light out of an incandescent bulb is to put more power into it. That's the bottom line.
I used an 80W/100W H4 bulb for quite a while, but the R100 alternator was hard pressed to keep up with it, especially around town. That's not a problem with K bikes, but the excess heat certainly could be. I did add headlight relays, and that helped with the light output and protected the headlight switch and original wiring, but stilll had problems melting the headlight socket on occasion.
Finally, we have the real solution: LED headlights. I've put them in both my Airhead and Brick, and the difference is quite noticeable. The H4 bulb puts out 1500 lumens (with small variations by brand), while LED's commonly put out 3000 lumens. That's a significant difference!
The beam pattern is, as others have pointed out, not the same. It's similar, but the sharp cutoff at the top of the low-beam pattern is a bit fuzzy, and the high beam isn't as narrowly focused, but the difference to me is not enough to matter. The brighter light rules, and the current draw is so low that you don't even need to consider adding relays. The ones I'm using pull 30 watts, half the power of the stock bulbs.
That's especially important on my Airhead. No more eschewing the electric vest around town for fear of draining the battery!
