Even though the novel doesn't explicitly state Genco's skills as consiglieri, the fact he was at Vito's side through at least two wars (versus Maranzano to establish power and during the time when the Irish guys shot Vito, when Sonny made his bones), possibly a third (the war with the sixth unnamed family whose Don Luca killed and subsequently sued for peace) and countless other instances that we aren't made aware of (favors for people, establishing and running the rackets) should say something to his skills since when we are introduced to the Corleone family, they're the most powerful crime family in New York. Clearly at the least Genco was better than the consiglieri's of the other families ;-)