You're absolutely correct. Even though sitting at third base gives you more cards to base your decision to hit, double, stand, or split, you can only make the best decision possible for the particular moment in time no matter where you sit. A correct decision to double down at first base may be taken away by the time the dealer reaches third base. You can only make the best decision available at any given time.
But what about the observation that positive counts generally do not appear until later in a shoe? Doesn't that give a slight edge to the guy at third base, since he is more likely to see a positive count where he can ramp up his bet, than if he were sitting at first base? That would have some effect, or am I wrong about that?