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Unibodys are extremely flexible, and that is generally frowned upon. Even with subframe connectors, the chassis is still overly flexible for high-end performance. The springs should be doing the work, not the chassis. A properly designed full-frame will greatly increase strength and solve that issue.
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I'll agree 100% that a rigid chassis is paramount for high performance, but I'd really caution against making a statement that a full frame dropped into a uni-body car is stronger than a unit body car with a good set of sfc's. It is really going to depend on how each is executed. The main nasty that shows up in a performance aspect is torsional rigidity, and in and of themselves, neither a ladder type frame, or a unit body car are very good.
However, the unit body cars can become really darned good with the application of a proper set of SFC's, such as the DSE through floor units. The reason is that while a full frame is bolted to the car with eight or so locations, and rubber bushes, the unit body car is welded to all of the frame elements. As such, all of the bent up sheetmetal, i.e. the floor pan, back seat brace, roof, pillars, everything becomes part of the structure, in three dimensions. Still not adequate for ripping up the road course with the C6R Corvettes, but not crap by any means either. All of the those little bends, divets, wrinkles, risers, etc in the factory sheet metal integrate and form a pretty darned stout structure. Compare a tin can with a smooth barrel, to one with the ridges in it, big difference!
Now, in the case you describe, the frame "would" be integrated into the body, much like a unit body car is, but depending on the floor that goes in, still might not be as torsionally rigid. Torsional rigidity is more dependent upon the design of a structure, than it is on the wall thickness of tubes used, take a look at a Maserati birdcage for an extreme example of frame-triangulation. The only way to know for certain if it is indeed more torsionally rigid is to measure it directly, it is super hard to model in FEA, for instance. I'd really be interested in seeing the difference.
Take a look at a contemporary World Challenge race car, those cars really heavily on a rigid chassis, and it is all done through the roll cage. Every tube on there is there for a reason, and it's all about triangles. Most of those cars are unit body cars.
I take nothing at all away from the frames Art makes, they are really nice works of art, really nice quality, and they clearly work pretty well.
Mark