How are you modifying the spark advancer curve?
In the beginning (...sometime AFTER God created the Heavens and Earth...) the 500/550 came with too-soft spark advancer springs. After a very few miles, this problem got worse from heat-annealing (which we see on all of these bikes) and the advance goes full by about 1500-1800 RPM. This is far too soon for the long intake tracts on these bikes: it causes spitback into the intake tract and reduces the low-to-midrange power by pushing the fuel charge back up to the carbs. This has always made them listless at engine speeds below 3500 RPM.
It takes a little tinkering to get it just right, because there are 3 different spring sets and advancers on these Fours. I usually start by cutting off 1/2 turn of one of the springs and making the shortest-possible loop of the new end to make it stay on the post (or weight), then ride it. When it comes out just right, there is a smooth, even surge of power from idle through the 4000 RPM range that, if you are familiar with that bike, you will notice right away: it loses that "switch on/switch off" power feel (at 4000-ish RPM) of the Mid Four when you get it right, instead giving a throttle that feels more 'normal' to other bikes.
Generally, overall they also have too much advance. This can be tested first by using lesser octane. If it makes them feel stronger at roll-on hiway speeds (like 4500+ RPM in upper gears) then it has too much advance to be running premium fuels. I personally think this was the reason these bikes ended up with #7 heatrange sparkplugs instead of #8, when it was an even higher RPM engine than the 750: the excessive advance makes the engine run too cool at 4000-7000 RPM speeds, which saps power (but it does improve engine life if the fuel is also too rich, hence their tradeoff). The choice was either to change the Owner's Manuals to show Regular Gas (which made the Honda Warranty department nervous), or increase the head heat to improve high-end power and sparkplug cleanliness, which was also Kawasaki's approach at the time.
The volumetric efficiency of the head design of the 500/550 is a little bit low, so it doesn't mix as well as the 750 or 350F/400F heads. This lends itself well to nice improvements with minor changes, though: it was a difficult head passage to cast, given the production technology of 1970. Because of that, it more resembles the venerable CB100 engine than the higher-performing 90cc or CB/CL72/77 series engines where the 750 was born. The CB100 has long been an indestructible engine (much like the 500/550), but just a little low on low-end torque. With some relatively minor tweaks, the incoming charge can be made to tumble less and flow more evenly, taking full advantage of the early-opening cams of the CB500/550K engines (the F engines have later-opening cams). Using a CB650 cam in this situation will improve things even more, so long as you don't mind losing the OEM tach.
