In theory, a straight pipe can help MPG but not by a noticeable amount. In practice, it noticeably brings it down because you’re always driving everywhere with your foot to the floor to hear the exhaust note. Been there, done that.
There are many, many factors that can introduce a rich running condition, you don’t need a tune be it a remap or whatever. Running rich can cause pops and bangs in the exhaust but it’s not always a good thing. You will end up with a melted catalytic converter and spark plugs so black they won’t ignite the mix properly under normal driving conditions.