You know, one thing we talked about a little while ago (don't really remember who was involved in the discussion) is that I don't really think ERA+, in spite of being league-normalized, is really an era-independent stat. The reason for this is that there just isn't as much room for the most elite pitchers to improve in low-scoring eras as there is for average and below-average arms. At some point, your handful of mistakes will cause a few runs to be allowed at any time. It's hard for anyone's ERA to ever really approach 0 over a substantial sample size.
Consider this: Pedro's ERA over the 7-season period from 1997 to 2003 was 2.20, good for an ERA+ of 213. This is likely the best 7-year pitching stretch in history, I don't debate that. What I might debate is the extent to which it's the best. A league-average pitcher during those years had an ERA of a little over 4.4. Let's compare this to Sandy Koufax's 5-year stretch from 1962-1966. During this stretch he had a 1.95 ERA and a 167 ERA+. An average pitcher in this era had an ERA of around 3.5. An average pitcher normalized into pitching home games in Dodger Stadium had an ERA around 3.3. In order to put up the same ERA+ during that time period, Pedro would have had to maintain an ERA in the neighborhood of 1.5. Put him in 1968 and he's have to be a little bit better than that.
Do I think Pedro could put up a season ERA better than 1.5 in the mid-60s? Sure, lots of people did. But I have significant doubts as to whether any pitcher who ever lived could have maintained an ERA around 1.5 for 7 years from 1962-1968. Maybe Pedro would have been better than Koufax or Bob Gibson. I'll give him credit for that. But that much better? Knock almost 25% off Koufax's 5-year ERA when everybody at the time thought Koufax was the greatest thing they'd ever seen? I doubt it. I just don't think it's reasonable for people to consistently put up ERA+ numbers of 180 and above in eras with league ERAs in the 3s. This is what makes Walter Johnson so impressive, and it's what makes me question the size of the gap between guys like Pedro, Clemens, and Grove and guys like Gibson, Seaver, or Feller. I mean, do we really believe the gap between an average pitcher and Pedro or Grove is over twice as big as the gap between an average pitcher and Feller or Spahn?