I've had a few CFL partisans insist to me that players in the Canadian league were as good or better than those in the NFL up to the sixties. Is there any basis for this? (And if so, could it have something to do with the CFL being more open to African-American "skill position" players?)
No. The CFL and NFL/AFL occasionally played exhibition games from the 40's through the 60's and the NFL was undefeated in those, usually winning by a healthy margin; the only CFL win was a 38-21 victory by Hamilton over (AFL) Buffalo in 1961.
There was certainly more parity at that time; salaries in the two leagues were much closer than they are now and CFL teams could occasionally lure players away with better contracts. In 1954, for example, the first-year BC Lions built their defense around pro bowl linemen Laurie Niemi and Arnie Weinmeister, signed away from Washington and New York. But even wage-competitiveness and the CFL being a little more welcoming to African-American talent wasn't enough to ever put the two leagues on an even footing.
More on the history of interleague games here: http://www.cflapedia.com/history/nfl_cfl.htm
in terms of individual players you could probably find a few that were as good as the best in the NFL. WE had a terrific high school player: Cookie Gilchrest who was playing for the old Cleveland Browns at age 18; he then went to Canada and played there for a number of years, finally coming to the NFL in Denver late in his career. Even as an old guy he set the NFL single game rushing record, I think in 1966. This record was broken by OJ Simpson in 73 I guess.
The example is that this guy never played in the NFL in his prime (Cleveland was in the old All American conference) and yet he had enuf talent to play there. So there were pro bowl level players in Canada, how many I am not sure, but it someone said they had a few players as good as any in the NFL I would not doubt it.