Except Luffy and Zoro don't spend 100% of their time training and fighting. Luffy has been shown, on-panel, fishing, laying around being bored, and goofing off in general with Usopp. This same amount of time Luffy spends goofing off instead of training is the same as Sanji spending time cooking instead of training. Time Zoro spends sleeping and cleaning his swords rather than training or fighting is the same as Sanji spending time cooking instead of training.
This "Sanji is the cook so he isn't gonna be as strong" thing only makes sense if Luffy and Zoro do nothing but train if they aren't in the bathroom, eating or sleeping.
ALL of their times are divided. Just because Sanji does so with a specific job doesn't mean the others aren't too.
Zoro is the only shown to be legit devoted to training on the ship.. He is physically superior to Sanji and anyone else on the crew.
If you find it is smart to compare 2 guys who only fight, to a guy who's main purpose on the ship is to cook logical go ahead.. Or if you find it logical to compare the 2 men that have goals that involve fighting as the main objective, to a guy who's goal involves exploring and "Finding" , then go ahead.
But its far from a rational thought, and a highly dumb thing to do, that only leads to Sanji fans overrating Sanji, or Zoro fans bashing Sanji.
I don't have it in me to do another Zoro vs Sanji, as the whole thing is dumb. Just my 2-cent on this dumb fan argument, that both fanbases should be able to understand.