Well for one, Sommers is losing to Gaban (or Sanji, but likely Gaban) while Shamrock will lose to either his brother, Sabo, or Dragon. So the fact Shamrock loses to people stronger than Gaban already kind of suggests he's better than Sommers (similar to Diamante losing to Kyros while Vergo lost to Law, or Queen losing to Sanji while King lost to Zoro). Plus he has a Mythical Zoan in his sword which is likely awakened, and considering his father and brother are top tiers, I seriously doubt he isn't also one lol