The title is the question. Please argue as much as possible about this, thanks.
In theory, I can only see fuel consumption improving...
But like others have said, once you enjoy the sound, you'll be tramping heavier on the accelerator, so fuel consumption will increase.
I have a dowpipe upgrade and resonator delete (stock muffler maintained), with a CAI and custom tune for performance (among a few other common upgrades).
I have gone from 9.5L per 100km to 11.6L per 100km with the various performance upgrades done... Sorry, but I am in South Africa, hence the L/KM instead of M/Gal etc...
But I would assume most of this change is due to the software tuning and a heavy foot. Stock motor/ECU with just a downpipe and muffler delete, I can't see too much damage to m/pg.