Koufax had five seasons (age 26 though age 30 seasons) where his ERA was sub-3.00, and in each of those seasons his ERA led his league (if not all of MLB). In three of them, he also led in wins, shutouts, and strikeouts, and in two of them he led in complete games and innings pitched. He won the Cy Young in three of those seasons, the MVP once, and was runner-up in MVP voting two other times.
Jim Rice and Dale Murphy are not good comparisons. Koufax in his dominant seasons was WAY more dominant than Rice or Murphy were in their four or five best seasons.
A good way to look at this is by ERA+ and OPS+. These are designed to measure a pitchers ERA and a batters OPS (On-Base Percentage + Slugging Percentage) compared to the league average, so that a score of 100 means you are league average and a score of 150 means you were 50% better than league average. The beauty of these stats is that they allow us to compare how dominant players were using widely accepted measures (ERA, and OPS) versus the the average player of their era, which gives you a decent method for comparing how dominant players were, across eras.
Koufax's top five ERA+ seasons (those five I already mentioned above) were 190 (CY, MVP-2), 188, 160 (CY, MVP-2), 159 (CY, MVP), and 142.
Jim Rice's five best OPS+ seasons were 157 (MVP), 154, 147, 141, and 136.
Dale Murphy's best five OPS+ seasons were 157, 152, 149 (MVP), 149, and 142 (MVP).
Murphy's five best were
mostly consecutive - 1982 through 1985, and 1987. Over the six-year period (including 1986, when he posted a very good but not great 121 OPS+) that encompasses all of his best five years, Murphy posted a 145 OPS+.
Rice is harder to peg, because his best years were only consecutive in the first three of his best five (1977-79), then 1983, and 1986. If you look at his OPS+ for the entire 10-year period that encompasses all of his best five years, he posted a cumulative 135 OPS+. But breaking it up (1977-1979: 153 OPS+; 1980-1983: 129 OPS+; and 1984-1986: 124 OPS+) might paint a picture that is a little fairer to Rice and a more accurate career arc.
In contrast, Koufax's best five year period
did come in consecutive years (and as PGStaff points out, he retired at his absolute peak at age 30, right after he posted that 190 ERA+), and Koufax accumulated a 167 ERA+ over that consecutive 5-year period. During that stretch, Koufax pitched in 181 games, won 111 and lost 34, with 100 complete games and 33 shutouts, struck out 1444 batters, while compiling a 1.95 ERA. The seasonal, per 162-game average for that time period was 22-7, 1.95 ERA, 275 IP, and 289 K with a 0.926 WHIP.
To put that into perspective, only 33 pitchers since the dawn of the 20th century have ever had a
single season that was in some way equivalent to what Koufax averaged for a
consecutive five-year period. The link above shows a B-Ref report for all single seasons in which a pitcher posted 20 or more wins, 10 or more losses, with at least a 165 ERA+ and a sub-1.00 WHIP. You can only sort by four categories, but if you required 250 or more strikeouts, that list would dwindle to 11 pitchers, and to 9 if you also required at least 250 IP.
And if you required pitchers to match or better Koufax's average during that 5-year period (>= 22 wins, <=7 losses, >= 167 ERA+, and <= 0.926 WHIP), there are only five pitchers who've ever done it in a season. If you also require at least 275 IP, only two (Walter Johnson in 1913, and Christy Mathewson in 1909). If you also require at least 289 K (but not 275 IP)? Only one - Pedro Martinez in 1999. If you also require
both a minimum of 289 K
and 275 IP...no pitcher has ever had such a season. That's how historically great that five-year stretch was.
Rice and Murphy were clearly dominant in their eras, but no matter how you slice it, neither of those dominant hitters' most dominant periods was nearly as dominant as Koufax's five year period between 1962 and 1966 was.
And yes, IMO, that is plenty dominant enough.