I think the only real world lab we have to test the quality of Negro League play is what happened in the first ten years of integration.
ML ROY 1947: Robinson
NL ROY:
--1949: Newcombe
--1950: Jethro
--1951: Mays
--1952: Black
--1953: Gilliam
NL MVP:
--1949: Robinson
--1951, 1953, 1955: Campanella
--1954: Mays
--1956: Newcombe
--1957: Aaron
Notable black players who came into the game from 1947-56 and had an impact at the MLB level:
--Jackie Robinson
--Campanella
--Mays
--Irvin
--Minoso
--Doby
--Banks
--Aaron
--Frank Robinson
--Clemente
--Elston Howard
--Jim Gilliam
--Newcombe
--Joe Black
--Hank Thompson
--Luke Easter
--Satchel Paige
Probably some others who don't come to my mind readily (I usually see their cards in my head and remember who was who that way). There were also a number of NL players who got very short trials in MLB and were cut down immediately if they were not spectacular off the bat. Mays was one of the lucky ones in working with Durocher, who was not quick to pull that demotion trigger on him after he went 1 for 25 to start, yet the Giants kicked Artie Wilson back down to the PCL after 24 at-bats produced 4 hits.
My point is that the black players who entered the Bigs in that first decade comprised an all-star team that could have beaten any white team of the era. Carrying NL stats as MLB stats, I don't see a good argument for not doing that given the quality of the players who were or would have been in the NL had there not been integration. Bottom line for me is that if NL stats are MLB stats, you can't make distinctions between seasons given how the game was played at a time of segregation. The NL players played the game they had available to them.
Oh, and a card:
