Are you sure?
I think 5.5 W/kg is way, way outside of the realm of amateurs. Have a look at the statistics across TR’s user base: using the graphical Bell curve calculator from this post only about 0.03 % of TR athletes have a relative FTP of 5.5 W/kg or above. (For comparison, I peaked at 4.7 W/kg last season, which puts me in the top-1 %.) The average is close to 3 W/kg. And this data only includes TR athletes, and I reckon that if we were to include more average people, we’d see even smaller numbers.
The share of potential customers for which such ridiculous gearing is suitable is really tiny. And regular road gearing (including a compact/compact equivalent setup) is overgearing for the vast majority of customers. Instead, in every situation I can think of, I would advise people to prioritize their lower gears rather than the other way around. There are plenty of prolonged climbs that, when I do them at about 3.5 W/kg (= top of my Z2), I need to go as slow as 55ish rpm on my lowest gear, 42:36 = 1.17 = 34:29. Most people would probably want to have three additional climbing gears.
I have 42:10 on my aero road bike, and I only spin out in situations when I would basically spin out with any gearing, i. e. prolonged, steep descents. On things like crit races, which are essentially flat where I live, I don’t ever touch my top gear. At speeds of 38–50 km/h, I’m typically in 42:12–42:14 (with a self-selected cadence of about 95–105 rpm). On hillier rides I use my 42:11 much less than either 42:12 or 42:10. The latter is my overdrive gear. (I know this, because I have an electronic groupset and it plots gear/time and gear/power diagrams.)
Have you done the math?
In my experience, pedaling becomes a bit tedious once I exceed about 58–60 km/h, which comes out to about 110 rpm in my top gear. My legs don’t like the combination of the light load (= low power output) and high rpm. I’m not a sprinter, so I can’t go that fast (= bike speed, not cadence) for long, unless I go downhill. On prolonged downhills, the limiting factor isn’t gearing, but bike handling skills (!), traffic (!), terrain and road conditions. I’d spin out any gear (including 52:11 or higher). On (false) flats, even in a group with fair wind, I rarely go faster than 55 km/h.
Even if your self-selected cadence is 90 rpm, you’d still be doing close to 50 km/h in 42:10 or 46:11. That seems plenty fast for most circumstances, including the races I have participated in.