How's this for a comp for Chapman?
Blue was a southpaw power pitcher. He possessed a breaking curveball that he threw on occasion and an above average change-up, but his signature pitch was a blistering fastball that dialed up to nearly 100 miles per hour. In The Neyer/James Guide to Pitchers, all-time hits leader Pete Rose stated that Blue 'threw as hard as anyone' he had ever faced, and baseball historian Bill James cited Blue as the hardest-throwing lefty, and the second hardest thrower of his era, to only Nolan Ryan.
Obviously he doesn't have the curve yet but does the slider.
Maybe Steve Carlton as another comp.
Steve Carlton went 27-10 with a terrible team one year in there, throwing high fastballs (the slider didn't become his best pitch until two years later). We have quotes about Steve Carlton when he first came up, comparing him to Sandy Koufax-but, since he is Carlton, those kind of quotes don't even make the file for him, because there is so much other stuff. Which is better evidence about the quality of his fastball: 27 wins, 300 strikeouts, or some guy thirty years later talking about what an awesome fastball David Clyde threw that one afternoon?
Not putting the bar high or anything.