I would caution that the MPG meter on the vehicle can be quite inaccurate. Resetting it mid-tank could give you a bad indication. I've had the meter give a spot-on reading on one tank, overstate by 2 MPGs on another, when doing manual calculations.
I don't rule out that switching off TCS will improve gas mileage or provide smoother shifting. I'll try it myself at some point if I can remember to turn it off consistently. But you have to be a little more scientific about it for a firm conclusion.
Step 1: Fill the tank until the pump shuts off. Do not top off past that point. Reset the trip odometer to zero.
Step 2: Drive with TCS on until the next fill up to get a baseline.
Step 3: Fill up at the same pump as before, again not topping off past auto shut off. For those who are in the habit of topping off I'm sure you've experienced one pump allowing an extra half gallon or more while another pump will allow hardly any additional.
Step 4: Manually calculate the MPGs for that TSC-on tank. Don't rely on the MPG meter. Reset the trip odometer.
Step 5: Drive on that second tank with TSC off under the same mix of city, suburban, highway. Somebody suggested a Post-It note as reminder to turn it back on with each start-up,
Step 6: Return to that same pump again and do not top off.
Step 7: Manually calculate the MPGs on that second tank and compare to the first.
That should eliminate most of the misleading variables.