If you look at a dyno plot of a stock duratec or SVT you can see that the switching point is pretty good stock. If you have a modified UIM or cams or something of that nature I'm sure the switching point would probably need to be moved to be ideal.
Your other option is to remove them totally. I've just recently played around with it by opening my secondaries at 1500 RPM so they are open most of the time and there's some low end loss I suppose but most of the time it's not even noticeable.
