Lately I have been trying to get my mind around the issue of protein versus gluten content, particularly with respect to 00 flour, where data on protein and gluten content is either unreliable or available only by going to the millers that mill the flour--and they are all in Italy. To give you an example, 00 flours are very fine in texture, actually talcum powder-like. This can mislead one into thinking that 00 flours are low in protein. In some cases, that is true, with some (including King Arthur's 00 "clone") coming in at around 8.5-9% (and the jury is still out on the Bel Aria brand of 00 flour). But, there are other instances, as with the Caputo brand of 00 flour, that have protein contents of around 11.5-12.5%, hardly a low-protein flour. I have been in discussions with the importer of the Caputo 00 flour (which, BTW, is widely used by pizzaioli throughout Italy and is considered to be the Cadillac of 00 flours), and part of the explanation appears to relate to the type of wheat, the quality of the wheat (which is often beyond the control of the miller, because of Mother Nature), and the milling of that wheat into what is called 00 flour. In Italy, flours are graded based on the degree of milling--with designations from 00, 0, 1, 2, etc.,--and not on protein content as it is in the U.S. So a 00 flour can have almost any amount of protein (within some prescribed limits under Italian law, I'm sure). Also, a good part of Italian flour comes from soft wheat. When Italian wheat is milled, the result is a weaker flour with elevated starch content and a diminished insoluble protein content (aka gluten). The fiber content is also among the lowest for all flours and the ash content, which seems to be of greater interest to European millers than U.S. millers, is similarly low. It is because of the low protein content generally--and possibly supply/demand issues--that Italian millers import flour from the U.S. and Canada, and combine the imported flours with their own to boost the protein and gluten content.
I think you are likely to find that the nature of the Giusto flour turns on the type of wheat being used, the tightness of the specifications dictated by Giusto to its millers as to the types and quality of the wheat it wants for its flours, and the milling processing used to achieve the desired end product--in terms of protein and starch content, ash content, fiber content, etc. If it has a lot of starch and little fiber, it will feel something like the 00 flour. If that is in fact the case (and I am only speculating here), the dough made from the flour should have extensibility but not a great deal of elasticity and will produce a soft and tender crust, and will be light in color as is also the case with the 00 flours. If I had to guess, that is what Chris Bianco may be striving for in combining flours, since he wants to replicate Neapolitan style doughs without having to import 00 flour from Italy. If it turns out that the Giusto flours you are testing have both extensibility and elasticity to a significant degree, they wlll likely have a higher protein/starch ratio and will be like most other bread and high-gluten flours in terms of the final product.
I hope soon to take delivery of some Caputo 00 flour and some flours produced by Keith Giusto to play around with. So I think (and hope) that there will be ample opportunity to speak with Keith Giusto and the Caputo importer to ask the kind of questions you are asking and I have been thinking about as well.