Ghost, no we do not know how strong a rider they are. I do know that when riding unloaded on my 50\39\26 bike, I stay in the 39 ring until probably 40kph, maybe a bit less, especially with the 12,13,14,15 cogs close together, which is just enjoyable to use with tight shifts.
but as you say, the only person who knows is the person in question. Strap 20 lbs somehow into the road bike and see how it is.
But don't forget, I think it's highly unlikely that a Rd can handle a 50\34 and a big old pie plate of 50t cassette.
Do the research and get back, I'd be interested to know what works, what's available and at what cost.
And I still say that the scant few times we can sustain speeds over 50kph downhill are pretty rare, so imo with touring, it's always a priority to emphasize low to mid speeds, cuz that's where we usually are, in kph probably 15-30kph, so smaller cranksets easily cover that.
And if you have the inclination, conditions, tailwind and downhill, you can still set land speed records going down big hills and around corners fast. I love doing this and gearing doesn't hold me back.
I use a 53/39 or 50/34 and 11-32 11 speed and absolutely never use the small ring at 40 km/h. My touring cadence is around 70-72 rpm. Spinning a 39 chainring at 40 km/h is very inefficient, metabolically and mechanically. I am not sure why you are talking about 50km/h.
As far as doing some research, no thanks. You do realize one does not need to use every ratio on the cluster, right? Or that the unpublished chainwraps are very conservative, right?