interesting. never heard of that... what that does is basically tune the intake runners for two separate operating conditions.
if you are familiar with the TPI systems on 80's camaro's, firebirds, and corvettes, you will understand this concept perfectly.
the TPI was a highly tuned intake system famous for having incredible low end torque but falling on its face right about at 4200 rpms.
what the valve does is allow you to have an intake tuned for low end torque that switches to a high velocity tuned intake above 2600 rpms.
usually when you tune an intake it is for a certain RPM band and the performace suffers everywhere else.

but i digress.

what it comes down to is this.
if you are driving around at 2600 rpms regularly then i would expect your mileage to suffer greatly.

as far as i am aware, O2 sensors have no way to sense backpressure. backpressure also will not affect your O2 reading significantly.

make sure the muffler is not on backwards.
make double sure that you are not driving more agressively.
2600 RPMs qualifies as agressive, so watch your foot if you are regularly in that RPM range

at this point, if it bothers you this much that your mileage has gone down, the simple solution is to go back to a stock muffler.

you could try a tuner with a mileage tune, but im afraid that you will pay more for the tune than you will save with the 1-2 MPG you will gain. generally speaking it is hard to gain more than 10% or so with a tune alone, especially on a stock motor.

if you had a bigger cam and larger injectors, or big tires and a re-geared rear end then i would be urging you to get it tuned, however.






edit:
sorry, im not trying to turn this into an argument. merely trying to help you get to the bottom of the problem.
in my opinion in cases like this, the problem usually lies with the "nut" that connects the seat to the gas pedal... in otherwords, the driver.
little joke there. no offense meant.