Interesting, so I found a shop to test ride a SL5 in 58cm. I was measuring 2 3/8" between the stays, or 60mm. There is definitely some healthy space in there!
Unfortunately, I didn't really like the isospeed. I found I would get a bounce in the saddle at certain cadence/power levels. Making that tunable would be great. The ride was very smooth though and if I was looking for something a bit more toward the touring/adventure side of things I would probably go with the Checkpoint. Ultimately, I ended up going with a 2018 Warbird. Less weight and fewer parts I'd be looking to swap out with time made the decision for me.