I don't have many concrete opinions about this yet, but don't you think it's odd that most of the very dominating performances in terms of % of games won come 1970's and 1980's? The general difference isn't even slight; it's huge.
I think these stats are for the open era only. There are other interesting stats on this page...I haven't finished looking through them yet.
edit: Oh yeah, of course not all of these are slam victories, but many of them are.