There's multiple statements of Kizaru being wary of him and even one with Beckman being stated to be comparable to Shanks.
(Databook Blue Deep)
Translation:
Always calm and collected, he is the number two of the Red-Haired Pirates with excellent situational judgment. During the Summit War, he checks on Kizaru. Even the Navy Admiral is wary of his fighting ability.
(Vivre Card)
Translation:
The Red-Haired Pirates reign in the New World. Standing beside Shanks as vice captain is Beckman. If Shanks is the “sun,” with his cheerful and lively personality and love of parties, Beckman is the “moon,” with his serene and dignified demeanor. He is the one who reminds the enemy of the horror of pirates and their true profession. With his experience gained from his long voyages, his clear mind, and his extremely high fighting ability, he is the equal of the “red-haired” pirate. His presence, comparable to that of “Red Hair”, makes even the fiercest pirates of the “New World” tremble...!
He has an eye that even the “light” can't resist! Even the Navy Admiral knows the danger of Benn Beckman!
(OP Magazine 15)
Translation:
He is cool, calm, and powerful. He is the spiritual pillar of the Red-Haired Pirates, supporting Shanks by his side. He is a Haki user and a strong fighter, so much so that even the Admiral, Kizaru, is afraid of him.
(Volume 4000000000)
Translation:
Does he surpass even the Haki of the Navy Admirals? Even Kizaru, who has a "Logia" type ability, cannot resist his checks and balances.
