I know for a fact that when Spider-man had the Venom symbiote it didn't make him stronger AT ALL. Not stronger, nor faster. Just gave him the symbiotic webbing. The idea of him being upgraded by the symbiote only appeared in the 90s cartoon.
Symbiotes grant humans superstrength but it all depends on how well the host controls it and how well they fit each other. In Spider-man's case he pretty much simply wore as a suit. So i guess it all depends on the host and the symbiote. When Carnage symbiote took over Ben Reilly it made him stronger but only cause it really TOOK HIM OVER. It was in controle. So I guess theoretically the Venom symbiote can make Spider-man stronger, but Parker would have to give in to the symbiote.. imo