Looking at the TFC-34. The RRS website is missing overall diameter and weight information. How would this setup work with the BH-40 head?
To further muddy the waters please realize that the fixed apex version was (probably first) designed as a tripod for precision rifle shooters (which I am) who don't need columns, leveling bases or ballheads as a photographer might. Instead they are marketing a newly designed head for firearms that is lighter and simpler than a photo head. It all works very nicely and like photographers, a PRS or hunter wants a light but sturdy tripod.
You can save a little weight and bulk with the fixed apex heads and now I see where you're coming from using the BH40 with the 34L legs but you'd have to try it and see. Probably only a handful of photographers are using it that way.
Also I wouldn't be surprised to see a lot of new ideas and products once RRS is settled in with their move. It's time to rethink how to lighten up tripods even further and I bet we'll see some lighter heads and more versions of the fixed apex design. So if you wait a year or so I bet there will be some pretty slick new ideas.