Sanji could, momentarily, become stronger than zoro.
Sanji's power, hell memories, is based off of his anger. The angrier he is, the stronger his flames are. This is essentially the same power that the hulk has from marvel comics. And if you've ever read marvel comics, you know that the hulk is stupidly overpowered for this exact reason. His anger is unlimited and hence his strength potential is unlimited.
We've never seen sanji go into a full rage. he's been angry, but we've never seen him straight up lose it. When he does, I expect to see some next level shit.
anyways I don't think Sanji> Zoro, rather I think zoro is probably significantly stronger. Just that under the right circumstances, Sanji could temporarily become stronger than Zoro.