For example, 2 valves from the modular 4.6 is equivalent to 1 valve from the LS1 which is a bigger valve.
Depends on the displacement of the engine and the size of the valves use, generally speaking four valves will cover more surface area than two valves. Using larger valves is generally cheap than using multiple smaller valves and less mechanically complex as well. But larger valves often need to be stronger and stiffer valve springs are needed, stiffer/larger valve springs = more energy required to open the valves which means there's a loss of power there, you can get around this somewhat by using stronger yet light materials like titanium, but if you do that, it can get expensive. And there's also the option of using multi-valve heads on pushrod engines like you see in the diesels used by the big three. To answer your question, there's no hard and fast rule about it.
OHC generally have 2 small valves instead of 1 large valve operated by a push rod. The aim is to have the most flow possible and the least mass of valve. (avoid valve float) I cannot say anything about specifics
Yes, because the largest 4 valves one can cram in the same cylinder head are smaller BUT provide more area than the largest two valves that would fit in the same head. With valves, the larger the overall area, the better.
