Strange. I wonder if issues like this can be traced to certain years. My K8 is all stock engine-wise and pretty much everything else except for my dyna S. I do not have the difficulties with rpms of speed or anything like that. I have not done the ton but that is by choice as I have kids and would rather see them grow then see a ton.
My coils are original too only change is the plug caps. I also have not done any real work to my wiring either. I wonder if later years have more powerful systems?
Yeah, I'm slowin' down...that's why my 17-tooth front sprocket is OK now (only...126 MPH at redline in 5th).
Here's how you can "test" the scenario: we used to call it "the tall gear wall", and it was noticeable from the K2 onward, in increasingly worse degrees to the K5: on a warm summer day, get into high gear at 40-45 MPH, then just roll on the throttle and see how fast it will go. Frequently, they would "hit the tall wall" at about 80-90 MPH and stop accelerating. If you then dropped into 4th and poured it on, they would break to the ton, and then would pull 5th to the top. On real hot (90+) days, it sometimes would not pull past the 4th gear's top speed, but would slow down in 5th (CB500/550 suffers this more often).
The problem is 3-fold, really: it's a subtle change in efficiencies from the K1 onward, starting with the pipes. To keep the plugs clean in slow driving, Honda had to lean out the (very) rich open-pipe mix of the earlier open pipes for the newer HM341 pipes. Rich mixtures fire much easier, and burn longer, at higher RPM (as any cafe' or RR rider has learned) with any given spark. With a leaner mix, the engine needs a longer (or wider-gapped) spark to ignite more of the flamefront to get it all to burn in time. If you then throw in the taller gearing of the K2-later bikes (18T front sprocket instead of 17T) and add the 2-row output bearing on the countershaft, the engine pressures rise enough to stall the flamefront in the leaner mixtures at wider throttle openings (2/3-3/4) and lower engine speeds (typically about 6500 RPM in 5th gear). This is where it appears: the roll-on creates a lean mixture just as the torque requirement rises, and the mix isn't rich enough to carry the cooler flame through the whole stroke, so it "stalls" at building torque.
If you add a wider plug gap and the voltage to jump it, a wider flamefront appears, which burns faster, hotter, and more efficiently. You can also make a longer spark, which the Honda coils are compared to the Dyna, to try to ignite a wider swath of the moving mixture. The Honda coils do have their limits, particularly if the points are burned somewhat or are cheapies (tiny mating faces make poor instantaneous peak currents), so the Dyna S units appear to prolong the "new points feeling" better after 1000 miles than did points. The Dyna coils discharge faster, so they need a wider plug gap to get that wider flamefront, and they have the volts to do just that.
The later carbs (late K4 and on) showed some attempts to improve the richness at higher RPMs to deal with this issue. Although fairly rare, some carbs came equipped with "lifting collars" like the ones used on 2-stroke big-bore port-timed engines (these carbs also have non-changeable jets!). These increased the suction on the needle jet at 2/3 throttle & upward, distinctly returning the "K0 feeling". I saw some of these just as I was leaving Illinois in 1974, on a pair of K4 bikes from California that I was tuning for some passers-by. These mods did, however, lower the MPG, to be like the K0's 35 MPG hiway numbers (the K4 was approaching 50 MPG without them).
So, for any given stock bike, improving the coils' performance turns the key to improved passing with No Fear on mountain roads. The "HM" Transistorized ignition does this by pretending to be perfect points, forever, through a little built-in electronic trickery. The Dyna S does it by being better than 1000-mile old points. The Dyna 3-ohm coils (with or without resistors) do it by heating the plug tips more if stock-gapped, or by widening the flamefront more with bigger (.050") gaps. The Dyna III does it by stretching the OFF time of the Dyna S-like trigger, to simulate the points' healthier dwell times. Any of these methods help fell the "tall gear wall". Richer jetting helps fell the wall, too, but with stock HM341 pipes, you'll pay for it in dirty plugs, unless you use higher voltage coils to keep them heated and burned clean.
One way to modify the carbs to get past "the wall" while keeping the in-town plugs cleaner is to open the mainjets and drop the needles, while at the same time installing low-restriction air filter(s). This 3-way mod will lean out the "dirty" range and reduce the in-town torque a little, but the personality shifts from Mr. Walker to Mr. Wheeler (remember him?) when you hit 6500 RPM. This is one of the things Honda did with the first "F" 750s, while improving the pipe and compression ratio. They also laid back the cam about 5 degrees, which enriches the top end, too. That's why the "F" feels like a rocket at top RPM, but the "K" tends to out-pull them with heavier loads. The "F" usually does a superior job of cleaning the plugs, as a result. And, some say, it's more fun to ride that way (right, RXman?).
