Great stuff!

I agree that heat load calcs are a good idea, working on a contact for that. I do have a contractor helping me but he doesn't have time for a lot of research. A few things that might or might not help clarify:

1) The stem walls are insulated but the builder's recollection was 1" foam, so R5 best guess. Per my earlier notes, floor is typically 3-4 degrees cooler at exposed edges vs center.
2) Basement is bone dry, thick plastic under slab and nice coat of gloss paint on the concrete.
3) Soil is most likely hard-pan but the house site is well drained. Sump has French drain and is always dry.

SO...if I could give up some height and get creative around my entry doors, how much foam under pex would start to make you more optimistic? What about 1/2" foam under the RHT system? I see that the RHT is based on 1/2" pex only. Would 3/8 pex be a possibility with some type of home-built set-up if I run the tubes closer or is this a non-starter? I looked at floating slab with either gypcrete or small stone concrete and this is possilble but big $$$. I was told I could glue some High psi foam and staple pex to that, then float a slab. In perfect world I would rather do a system like RHT but I am trying to look at every possibility.

One other thing...anyone ever try to use a router on foam to grove for pex? Was just thinking maybe I could start with some thicker foam, make my tracks and then either go with T&G floor over or float a slab, perhaps with heat plates like the RHT stuff. Maybe fools gold but figured I would get a direct answer here either way. Agreeing with other comments I know that forced air is a non starter for me and radiant panels would do well. All that said, I know that heated floors would be great if I can make it fit somehow. Thanks!