Bigger question is how Cobb would do against sliders and changeups and splitters and relief pitchers coming in throwing in the upper 90s.
Remember that the spit-ball was legal through most of Cobb's career. He hit pitches that would scare the wits out of modern ballplayers.
Remember that MLB tried to use a few baseballs as possible, something like one, in a game, except when Walter Johnson pitched. Johnson demanded a clean baseball because he did not want his fastball to jump in unexpected and sometimes uncontrollable ways. Cobb and the others faced dirty baseballs, scuffed baseballs, baseballs darken by mud and maybe unbalanced by water.
For details on the baseball see Hank Thomas, in his biography of his grandfather, where Hank explains that Johnson demanded "a clean baseball" because he was always afraid he might kill somebody. Johnson had such stature that umpires always gave him a new ball.
Remember, as well, that baseball was played in the afternoons, sunny or darkly clouded. With the dirty darkened baseball late in a game on a cloudy afternoon, Ray Chapman probably did not see the fastball that killed him. That was 1920.
There are always arguments about how hard earlier pitchers threw. It seems likely that they could throw very hard when they wanted to or needed to, but pitchers were expected to pitch complete games.
What did they throw? In my baseball lifetime, starting in the early '50s, a starting pitcher was expected to throw a fastball, curveball, slider, and usually a change-up. That must have been standard for a long time. Pitchers like Warren Spahn also threw a screw-ball, which broke the opposite way from a curve. Carl Hubbell broke into the majors in the late '20s, and Hubbell threw a screw-ball, so it was not a new pitch.
Roy Face threw a fork-ball, something that behaves a lot like the split-fingered fastball. First time I noticed a splitter was when Mike Scott used it to confound the Mets in the 1986 NL playoffs, but it must have been around. (Although I don't know the history).
Biggest change I have seen is the six-inning starter and the one-inning relief pitcher, although Joe Torre's Yankees routinely used Mike Stanton and Jeff Nelson and Ramiro Mendoza for the seventh and 8th innings, before Mariano Rivera pitched the 9th. Maybe its now the five-inning starter...