Not really sure what to make of Xebec’s strength yet. My guy Kong literally got promoted above Fleet Admiral, so his strength is definitely the real deal. High top tier in his prime for sure.
Xebec is a weird one. I don't think Wb fought against Roger & Garp because that would negate his legend and all the good values Oda installed on him in MF, so Traitorbeard is prolly out of the equation.
My headcannon is Roger & Garp beat the shit out of Meme and Kaido in a 2 vs 3 then ganged up on Xebec.
From the hype and his notrorious status, I can't see him lower than Primebeard, so at least stronger than Wb but weaker than Akainu, who at EoS will only be below Luffy and Teach
EoS BB > Eos Luffy > Sakazuki > Garp >= Roger ~ Xebec >= Traitorbeard