Seems to me that Sonny knew his limitations. In the novel, after Sonny makes his mistake at the Sollozzo meeting, Hagen is horrified. He thinks he will never play consigliere to Sonny's Don. After the Don is shot, Sonny flushes when he admits to Michael that he "shot his mouth off" at the meeting, and all but blurts out responsibility for the Don's shooting. Sonny also tells Hagen that he knows he's not the man the Don is, but when it comes to warfare, he's as good as anybody.