Just a couple of points about brighter headlights in the CB550.
1- While the CB550 and the CB750 have about the same electrical load in stock form. The 750 has a stronger charging system and a larger capacity battery. The 550 has a 150 Watt alternator vs. the 750's 210 Watts. What this means, is that the 750 is better able to tolerate additional devices that consume more power. The CB550 stock headlight, for example is 40W low beam and 50W high. When you replace the headlight for one that throws more light, it usually also draws more power. And, it is wise to consider the extra power or electrical load you are placing on the charging system. Because, the 550 alternator only makes charging power when the engine is turning 1500 to 2000 RPM and above. At idle it doesn't make enough power to run the electrics AND charge the battery. In fact, at idle with the headlight on the battery is draining and won't get to recharge until you rev it up past 2K RPM. If you add even more electrical load, the battery drains even faster. And, the engine must run even faster, say, 2500 RPM to overcome the added draw of brighter, larger wattage headlights. And, of course, because extra power is being expended in the headlight, less is available to put back into the battery so, recharging takes longer. What the effects of repeated draining and recharging of the battery are, is shortened battery life. A couple of my 550 bikes have higher wattage headlights. Battery life is probably cut by 1/3 on these bikes compared to the ones I have with all stock lighting. Also, the turn signal flash rates are slower to non-existent unless I keep reving the engine so the alternator makes enough power to keep the system voltage at peak levels.
2. All the electrical system connectors and switches in a 30 year old bike likely have some corrosion on them. The corrosion is resistive by nature and this resistance wastes power (as heat) and lowers the voltage and power delivered to the end device, such as the headlight. Consider that if there are 10 connectors and switch contacts in line with the headlight and each drops just .1 volts, the headlight only operates on 11 V not 12 V. This makes your 40W headlight project only 36W of power; a loss of 4W. This is noticeably dimmer and is not just due to the age of the original lamp. The problem is worse with higher wattage bulbs. A 70 W headlight operating on 11V instead of 12V loses 6W, effectively making it a 64W lamp. The second part of this problem is that while the connector corrosion lowers voltage, it doesn't change the current draw. The load determines that. A higher wattage bulb WILL increase the load and current through all those corroded, resistive connectors as well as the fuse contacts. All these parts will heat up even more with the extra current passing through them. The fuse clip contacts may even heat enough to start MELTING the main fuse from the heat generated, and the extra current passing through it for the new bigger, brighter, power hungry headlight.
My recommendation is that you measure the voltage drop between battery + and headlight. Also, measure the voltage drop between battery negative and headlight negative. Find out what your current headlight operating voltage is at the headlight. Could be you need to refurbish your electrical interconnection system to get proper voltage up there. If that doesn't satisfy your need for more light, then, at least, the system will be better prepared to accommodate the added draw of the new headlamp selected.
Make sense?
