If you even get any MPG improvement, it will take a long, long time to make up for the $250+ cost of the tuner.
As for performance, the 91/93 tunes are the ones that actually give you more power, but at the cost of higher fuel prices (making it even harder to justify buying one for improved MPG/fuel cost).
I have an 87 octane performance tune with street shifting (firmer shifting than stock) from MPT on a SCT x3. My total cost was $250 ($200 for a used tuner and $50 for the tune). My review on it
I see no MPG difference so far with mine (but only 1 tankful), but some with the higher octane tunes say they do have a little improvement (again, minimal gain, at the cost of higher fuel prices).
When I was debating what tune to get, everything I read about the 10th gen truck says that any "economy" tune is worthless.