My best guess is that my SVT is turning around 3700rpm at this speed and the secondaries are open, dogging the mileage
This doesn't sound correct. At 70, my SVT is at 3000 RPM's, at 75 about 3250 RPM's and at 80 about 3500 RPM's. The secondaries are not open at this point. They kick in at about 3900 unless you are at WOT, then at 3400.
It sounds like you have other problems with your gas mileage. Check the normal stuff, plugs/wires, fuel/air filter and tire pressure.
