1. https://www.bikecalc.com/speed_at_cadence
2. https://si.shimano.com/pdfs/dm/DM-CS0004-04-ENG.pdf

Next, for a 12-25 in 11spd you get 1 tooth jumps starting all the way up at the 19. Whatever front rings you choose, that's nice. A 50/15 at 90rpm is about 23.5mph. That's your first one tooth jump down to 14 for an 11-28 cassette. An 11-28 cassette is probably the #1 cassette coming on road bikes these days and probably seen on them.

Meaning, it's ****. I'm not a fan of the 11 or the 28 for "around town" club riding or even racing. Unless there's mountains, you're shifting big gaps or jumping front rings all the time.

You want your front ring changes to be very intentional and infrequent. Meaning, a good hill. You don't want to be shifting in and out of the big ring while going ~20mph just to be able to maintain 1-tooth cog spacing.

The 11t rarely has its place. In time trial I do a 54t with 12-25 and it's awesome being able to only ever shift 1 cog increments the entire race.

If you must do a 28t cog, the 12-28 gets you one tooth increments starting at 17 instead of at 15 like the 11-28.

If I were bike companies, I'd ship new road bikes with a compact crank and 12-28 cassettes.

Just me.

I could have written this response verbatim. This is what's on my fast road bike now. I rarely use the 34-32T combo, but am glad to have it for the off day.

I'm planning a carbon road build right now, and will buy an R8000 groupset for that bike in June. Yup, 50-34 and 11-32T.

I race bikes on a compact 50/34 with an 11-28 cassette. I have hit speeds of 50+mph and never, ever thought to myself "darn I could go faster if I just had another gear". At 45+mph, tucking in a low position with your head down and your hands on the tops (lower than I can get with pedalling, and definitely more stable) is WAY faster than pedalling. I can't even imagine how many watts you'd have to put out to make up that difference. Probably around 200-300W. Which is a heck of a lot more than 0W.

Most races and group rides aren't stupid enough to have their sprints point downhill. The insane speeds combined with the ridiculous slingshot is a guaranteed trip to the hospital. So don't worry about sprinting at 40+ mph. Unless you're a cat 1 crit racer. Most mortal sprints hit 37-38 tops. I don't think I even use my 11T cog for those speeds.

I have pedalled at 42-44mph and not spun out. We need to discuss what your RPM range here is. What do you consider "spinning out"? Personally, I grind around 50RPM on the steepest of climbs. I can sustain 100-110 rpm on climbs, with bursts of 130rpm for accelerations. Those high rpm bursts require easier gears.

If you're comfortable grinding on the few hills you have, I recommend a tighter cassette like 11-25 and a compact crank. If you're spinning out under 100 rpm, look at your fit. It doesn't require ridiculous fitness. Are you too far high or back, forcing you to reach for the pedals? Does your knee have a tick? Do you have a very large belly, forcing you to sit too upright? Is your saddle shape weird, causing you to sit with your pelvis rotated backwards? If your butt is uncomfortable, you won't spin fast because you will "stand" on the pedals to relieve your butt of pressure. Actually, if anything is uncomfortable at all, it will affect your ability to spin smoothly. Is your crank too long, causing you to use too much of your knee's/hip's range of motion and throwing your rhythm off?

Regarding fixed gear riding - I'm not sure if it will help more than strength training and experimenting with your fit. When I was riding fixed, I actually had a very choppy pedal stroke because I was so reliant on the flywheel effect. I was able to spin up super fast (155rpm max) on descents, but I wasn't generating useful power. My legs were getting dragged around and I was just along for the ride. My knees really hated that.

All this is good. I've concluded that compact + 11-28 is the way to go.

Working on my tucks and general descending skills is already helping to get additional speed downhill with my current gearing. I hit 47MPH the other day going into a slight headwind (and I am not a heavy rider.) I have an 11-25 laying around, but with quite a few hills, I like keeping the 28 as a bailout gear (most local riders use compact + 11-32).

I think your conclusions on fixed gear mirror my own; as I've gotten older, my knees do not enjoy fixed gear descents. The (non-fixed) single speed is turning out to be a useful training device - both for increasing cadence, and for descending skills. On a few of my favorites, a proper tuck is required to keep up with traffic; scrubbing too much speed means you've got to pull over and let the cars pass.