My wife and I stayed in Rocheport and rode the Katy over 4th of July weekend. I swapped the Gatorskins out for Gravel King SKs, prior to the trip. Just wanted to share my experience.
1. The bike was more squirrelly, and took some getting used to. Specifically this showed up as instability related to the
trunk bag that we have on a rear rack.
We normally ride with that bag on all our tandem road trips. On the Katy, with the Gravel Kings, I had to really pay attention in order to keep the bike well controlled. I did not feel comfortable riding one-handed, for example, and actually stopped whenever I wanted to drink or interact in any way with the bike computer. After a while I unzipped the side panniers on the bag, and moved everything into the panniers in order to lower the center of gravity. This helped, a lot.
2. We never got bogged down. Trail conditions were mostly dry, except for some dampness right along the river, for about 1 mile west of Rocheport. There were also several spots on the trail between Boonville and Columbia where the surface gets very fine, almost a sand. We floated through that with no issue.
3. Efficiency on road and trail was fine. I'm primarily a roadie these days, and geek out over how fast I can go, and for how long. So I place a lot of priority on efficiency, and the Gravel Kings felt fine. I never felt like we were being held back by the tread or width, on either asphalt or trail, compared to the Gatorskins that we normally run.
4. Lots of stones and grit kicked up. The area just west of Rocheport was a little damp, as noted above, and so lots of small particles stuck to the tires and were flung up. Unfortunately, this was the very first section we rode, on Saturday evening when we arrrived. Given that it was my wife's very first gravel ride, and we don't have fenders on our tandem, I heard lots of complaints for a while. She took a stone in the cheek which didn't help. I adapted by keeping the bike on the most highly traveled/packed part of the trail, which is about 6 inches wide. That helped, and it also helped once we got away from the damp section. We both had a layer of grit on our legs by the end of the ride. On days 2 and 3 we went east, and took the MKT spur up to Columbia once. It wasn't so bad there, and by then my wife had reset her expectations so it was fine.
5. Traction on loose gravel was good. Obviously the Katy itself is a good, well-maintained trail made of mostly crushed limestone, and traction was never an issue anywhere. The MKT trail, which runs from Columbia down to join the Katy, was slightly less smooth at the southern end closest to the Katy. I'm not talking mountain biking, but the gravel is looser there. I never felt any concerns about traction or handling with the Gravel Kings.
6. No flats. I had concerns about this and packed two extra tubes in our bike bag to be safe. We rode a total of 70 miles and had no issues. I didn't see any significant nicks or cuts on the tires, afterward.
Summary -- all in all, I think the Gravel Kings were fine. On days 2 and 3 my stoker said she enjoyed the ride as much or better than any ride we've ever done, even with the gritty shins. Having no wrecks and no flats, and never having to walk through the loose stuff, I'd say it was a success.