Jack Dempsey .Vs. Billy Conn

Brennan knocked out a few fairly durable guys in his time, even if they weren't great fighters, and he has a better KO ratio than Conn.

 

Yes, but the bottom line is Conn stopped three of the five top five heavyweights he fought (Pastor, Barlund, Dorazio)--There were no ratings when Brennan fought but I would doubt he ko'd anyone who would have been a top ten heavyweight. Also, Conn was only 22 or so when he moved into the heavyweight division. It is not necessarily surprising he began to pick up power. It has happened to other maturing boxers, such as Carlos Ortiz.
But rather than debating the relative power of Brennan and Conn, my point would be that there is no evidence Brennan had enough extra power over Conn, if any at all, to outweigh Conn's obviously superior skills. Brennan just wasn't that dangerous a puncher.