As far as deadball pitchers go, it appears that the more "realistic" K/9 numbers are only attained by dominant pitchers with the lowest ERC#s. This shows itself both in SPs and RPs in my data. Some examples...
'14 Dutch Leonard: 7.93 K/9#
Observed at 80M: 6.42 K/9
Observed at 147M: 5.34 K/9
'13 Walter Johnson: 7.15 K/9#
Observed at 147M: 5.63 K/9
Observed at 120M: 5.89 K/9
'15 Tim McCabe: 3.93 K/9#
Observed at 80M: 4.01 K/9
Observed at 120M: 3.57 K/9
Worth noting that the RPs numbers can, of course, vary more because of the smaller sample size that they provide. Regardless, the numbers still do not equal their real life K/9#, but neither do those of the modern pitchers I have data on. A quick example...
'95 Greg Maddux: 5.97 K/9#
Observed at 147M: 5.62 K/9
'13 Clayton Kershaw: 6.06 K/9#
Observed at 120M: 5.5 K/9
'68 Luis Tiant: 7.31 K/9#
Observed at 120M: 7.11 K/9
While even the elite deadball pitchers do not come quite as close to their K/9#, the non-elites, second-tiers, whatever you want to call them, perform much worse. Examples...
'06 Doc White: 4.27 K/9#
Observed at 80M: 2.1 K/9
'02 Bill Bernhard: 2.69 K/9#
Observed at 120M: 1.48 K/9
Observed at 80M: 1.86 K/9
'09 Harry Krause: 6.55 K/9#
Observed at 80M: 3.32 K/9
'06 Jack Pfiester: 6.36 K/9#
Observed at 80M: 3.49 K/9
As with anything else, there are also outliers even among the elite...
'08 Addie Joss: 3.82 K/9#
Observed at 80M: 1.77 K/9
Observed at 120M: 2.42 K/9
9/8/2015 8:40 AM (edited)