Your question:
how much more power to keep a 5 or 6 happy than it takes to power a 4?
Is actually an important consideration for all electric bass players.
Consider that a guitarist may be using a 60 watt amp and actually using 20 watts while playing two octaves above the bass line. Since one has to multiply wattage by a factor of 10 to achieve an equal Sound Pressure Level (SPL) for each octave down and 10x10 for two octaves down which is the real world ranges during a guitar solo.
In this case the bassist playing two octave below a guitarist using 20 watts would need 2000 watts. When you travel down into the sub-basement of bass playing add another x5 for the additional half octave further down. That is 10,000 watts. Obviously this is an unrealistic amount of power for a personal stage amp but illustrates the problem.
Of course if your guitarist has a 100 watt amp and uses all of them there is no point in even trying to compete, just get ear plugs and hope that your sound guy has at least 60,000 watts available for the sub bins. As you can see, it gets ridiculous.
This disparity in power available vs. power needed directly correlates to the history of bass amplification chasing developments in guitar amp, PA system, and drum technology. We bassists have always been at a loudness disadvantage and this trend will probably continue until some egotistic musicians get over having to be louder than others.
To answer your question. I use 5 and 6 string basses having low B strings. My most frequently used amp is an Eden WT-550 (550 watts @ 4 Ohms, excellent EQ controls) which has adequate power to drive a 4x10 speaker, barely. For louder, or outdoor situation I use a 2000 watt Stuart PM 2.1 rack mounted power amp.
Yes, you should have more power available when playing an instrument having a low B string. You should also have a speaker (or pair of speakers) that is rated at approximately 600 to 800 watts program and 1000 watts peak power handling and designed to operate down below 40 Hz. Most good quality modern 4x10 boxes are in this range.
There is another approach to living with the Low B string. Restructure your tone so that it is less boomy and more present in the 1st harmonic (about 80 to 140 Hz). This thinner sound can still sound full and have "punch". Such a tone avoids overdriving the speaker in the low end instead drives in it's optimum range, and greatly helps the fretless mwah sound.
Just because the low B string has a fundamental in the basement does not mean that one has to actually reproduce those very low frequencies with equal loudness as other slightly higher frequencies. This implies that you need a pre-amp that has good EQ control and will allow you to reduce the below 30 -40 Hz range and boost the 80 - 140 Hz range. This may actually help your fretless tone (assuming that you like Jaco).
One can use gear that is not massively powerful when playing basses having a low B. It is just a matter of not overdriving the amp/speaker and listening to avoid distortion. If this smaller gear is perceived to be not loud enough for the situation then think about getting a more powerful rig.
Another useful option is to let the house PA system do the heavy lifting. Use a decent D.I. to send your signal to the front of house mixer and put your smallish amp on a stand as a personal monitor.