You will like the upgrade. It is really awesome. Doing 70MPH and punching it to pass a car is a blast. The Trans downshifts with authority and you are going 85 in seconds. It may just be me but the sound is a lot lower and more powerful when you get on it.
The SLE comes with dual exhaust and the SE does not so your sound may be different.
Gas Mileage remained the same but I had my foot into the gas more than usual.
We took a different route and it seemed to be more mountains than if we went up I70 to Breezwood then on to Pittsburgh. We took I68 through Cumberland then picked up I79 in Morgantown. Took I79 to I76 west of Pittsburgh. This leg of the trip averaged 26.3 MPG. The rest of the trip into Detroit brought the average mileage up to 29.1MPG.
I believe the mileage to remain the same.