Maybe a few things to add:
* My WF in halcyon fits Eagle Creek folders:
Granted it's a bit tight, and I only carry one stuffed to the gills in the bundle method, but the stretchier halcyon fabric allows for more room; for heavy business travelers I'd recommend Halcyon over other fabrics as it's just more flexible and it's plenty durable in minimum room. In 3+ years of 25+ trips/year, under seats, in overheads, I have a few small marks that came off with a wet rag - very durable!
* Yes, the WF fits great under all seats
except very small / odd commuter flights, but it does fit in the those tiny overheads!
* I carry a ton of stuff in the WF:
3+ suit and fixins, full size BT speaker, shoes, big laptop in cache, dopp kit, and lunch!
I also use it on the absolute strap with an S25 as my support bag - people are always shocked those are my only bags and, really, I don't routinely need the S25, I'd just need a Pilot which serves as my briefcase and which I carry rolled-up in the S25 until I get to my destination
The WF is the unsung hero bag in the TB lineup - my only complaint is the weird fake-silk side handles ... I get they're probably better than webbing in terms of use and getting caught on things, but seems like it'd be easy to add Synik / TS type handles. For me, it prevents the bag from feeling premium