As I've explained before, I have Coveleski (or Kowalewski, the name I've been using in voting for him) clearly ahead of Mays. The comparisons I'm going to have a hard time with are Coveleski with Vance, Rixey, Faber, and Ruffing. Since Coveleski and Vance both have about 3000 IP, they're a pair to be compared.

They look incredibly evenly matched.

ERA+
SC: 127
DV: 125

INN
SC: 3082
DV: 2966.7

BLACK INK
DV: 66
SC: 22

GRAY INK
SC: 193
DV: 171

BEST P IN LG (BY WS)
SC: Never best pitcher in league
DV: 1924 (36); 1928 (32); 1930 (26)

TOP 3 PITCHER IN LG (BY WS)
SC: 1918 (29); 1919 (27); 1920 (32)
DV: 1927 (25)

BEST 3- AND 5-YEAR NON-CONSECUTIVE PEAK BY WS
SC: 90/142
DV: 94/139

BEST 3- AND 5-YEAR CONSECUTIVE PEAK BY WS
SC: 88/142
DV: 77/124

NEW WARP1/3
SC: 84.2/77.8
DC: 88.9/80

And there's two other relevant questions I think:
1) Why do both pitchers have several missing years early in their career?
2) Are there any league-difficulty issues to be aware of?

Vance had arm trouble early in his career, a sore elbow. As Bill James tells the story in the NBJHA, some random bump turned a chronic pain into an acute pain in the elbow and led to a not-well-documented operation on Vance's elbow. (James speculates it might have involved removing bone chips.) Somehow, after that operation, Vance could throw without pain and set about the career he had as an extreme strikeout pitcher.

I would treat Vance's missing years the same way I treat Joss's missing years, or Dean's missing years. It's injury or illness. He wasn't a great pitcher during that time because of the injury or illness. The fact that the injury comes before his career is a bit strange, but that's what it is.

Vance vs. Coveleski. Vance not only had far far worse offensive support, he also had worse defensive support. Also, I've dumped in Dr Memory's K+, BB+, & K/BB+ numbers on my site (too lazy to link - just go & click on the appropriate places to figure what they are) - Vance has the best adjusted K-rate & K/BB ratio of any pitcher ever.

I want to restart discussion of Mays (and to a lesser extent Wilbur Cooper). Both ranked in the top 5 pitchers in their league 7 or 8 times by most measures and put up at least 200 wins. Mays obviously had great help from his teammates, but also switched leagues at almost the exact wrong moment, guaranteeing that he was pitching in the tougher league for almost the entirety of his career. Plus any calculation of Mays's peak needs to project 1918 and 1919 out to a full schedule. On further review, I have Mays ahead of Walters (who's about 28) and behind (but not lapped by) Ferrell and Lemon (who I see as about 4th and 6th on the next ballot).

I'm a FOWC. He's solidly within my group of best 75-best-ever hurlers for the HOM.