Everything makes sense there. From what I have read there is no different in rigidity between different chromoly steels. Dent resistance and durability of higher grade steels should be better though.
As for the partial double over size design it seems like a decent idea to me, but if stiffness is indeed your main concern than going 31.7mm TT would be better as would larger seat stays. The seat stay diameter will effect the lateral stiffness of the rear triangle. How much stiffness the seat stays add is something I've been wondering about lately as tiny seat stays are quite popular, but then as a counter point to the possible benefits of small seat stays you see a Pegoretti and it has HUGE tubes in the rear triangle and read Silca's blog post analyzing where comfort comes from on a bike and the frame is the least important part of the equation relative to wheels and tires. So, maybe large seat stays would be great on a bike that is built to ridden hard and fast.
Are you building with lugs?