It would not have made any difference. The Greeks (the poleis resisting the Persians, that is) already had a sizable army at Thermopylae and were holding the pass quite comfortably; additional forces would have been unnecessary at that stage. Sure, you can argue that more Spartans would have meant guarding the goat path better, preventing the pass from being outflanked by the Immortals.
However, this ignores the fact that the Greek fleet was being whittled to death in the naval engagement at Artemisium. They were getting ready to retreat after the third day anyway. In fact, one might argue that having MORE soldiers at Thermopylae (and hence keeping the pass for longer) might have been a bad thing for the Greeks: the navy may have been browbeaten to continue fighting the more even engagement at Artemisium, and hence jeopardize the ability of the navy to win at Salamis.