I know there is a small amount of resistance, but not more than your muffler.The point I'm trying to make is "wouldn't the valve help a slip-on in the lower RPM range" and if any one done it? Actually I don't think it'll work. I think some one would have to make a slip-on to accommodate the valve because of the diameter of the tubing after the valve.
By the way, I have never heard of anyone polishing headers... I knowI port and polish heads and intake manifolds... I might be wrong though.

 

that is what i was getting at...And people buy after market headers for more direct airflow with less curves and smoother interior finish...btu anyways.

and as far as your question, yes if you were to add it, it would help on the low end, But you would loose on everything else. Which the whole reason you are adding a slip on is for the "everything else" You ride the bike around more in the "everything else" range.

If all you did was stop and go bumper to bumper traffic "all the time" Then it would be worth it. (but that isn't why we buy motorcycles is it?)

There are Pros and Cons to every modification, You gotta give in order to get. If you add the valve to a Slip on, you loose alotof the benifits of the slip on. To gain better airflow for HP and throttle response you sacrifice low end torque, but this is all justified because the bike has planty of torque to loose a little. You add the valveback into the equation, you get the torque back, but loose your HP and throttle response gains the slip-on originally gave you.