RE: Gen-5 architecture:
The only "sacred cow", here, are the bore centers @ 4.40". Changing those makes retooling/manufacturing much more expen$ive, and instantly obsoletes ALL that the previous Gen SBCs are/were based on.
That is where the limiting factor for "bore" comes in. You cast the blocks with 4.40 centers, and sleeve down from there.
Valve sizes: you don't build a 4.00" bore engine with 2.20"/1.60"+ valves..."shrouding" happens. Take a look at LS1/LS6 engines: smallish bore = smallish valves. Pretty much max'd @ 2.05".
Valve sizes, alone, do NOT correlate to greater revs. In fact, a smaller valve weighs less, and theoretically can be rev'd higher... That's part of the reason that a 4-valve engine revs higher than an equivalent 2-valve.
Port dimensions and shapes are important to high revs. Bottlenecks in the ports restricts your usable rev capabilities... More air in/exhaust out = more power-better emissions, again depending on port design, and combustion chamber shapes. Port velocity is more important than all-out port size.
Raised cam, as seen in the Ilmor/Chev cum Mercedes Indy engine, = shorter pushrods = less valvetrain weight = higher rev capabilities. NASCAR engines utilize flat-tappet cams, much less efficient than rollers, and still achieve 9,000+ rpm for hours on end. Almost impossible to do with current production-based long pushrods...
With DI, optimized bore and stroke per displacement, and optimized cylinder head design, combined with short pushrods and VVT, these engines regardless of displacement will offer power-to-size with emissions AND economy previously only dreamed about...