The cb550 only has a 150 watt alternator. The Cb750 has a bigger one, 175 to 210 watts depending on the year. So, the 750 is better able to handle the extra load of a headlight that draws more power. The 750 also has a larger battery giving it more stored power. This is important, because at idle, the alternator isn't spinning fast enough to supply all the electrical needs of the stock bike. And, power is drained from the battery during idle. A larger headlight will drain the battery faster at idle. Also, it will need a higher RPM to provide restorative power to the battery while driving.
Much depends on how you operate the bike. If you operate in the city and spend a lot of time idling, you could run the battery down significantly. If you spend most of your operation time on the highway in cruise, you probably won't see much of a battery rundown issue.
Something to think about with these old bikes is voltage loss through all the connectors that power the headlight. 10 connectors losing 0.2 volts per connector means your headlight only has 10 volts to operate on. This will make a 35 watt headlight shine with the brightness of a 30 watt headlight.
Also, increasing the current draw through such lossy connectors heats them up more as they drop even more power when carying higher currents. You should measure the voltage right at the headlight terminals while it is operating to see if you are getting the full battery voltage to it.
Lastly, the main fuse's operating margin is dimished, making it run hotter. Any fuse clip corrosion will generate even more heat and it will conduct to the fuse element. They combined heating effects can melt the fuse without a true overcurrent condition.
good luck,
