I have only ever messed with timing, you are never going to gain MPG in the city unless you light foot it everwhere. I am lucky to get 16-18 city if that.
Basically I took a 04 GTP timing table and did some voodoo magic and combined it with my 00 GTP table for getting better freeway MPG. I have no cat or egr, so raising the timing a few degrees in all the spots (cells) the car would see during freeway driving helped. Stock is kinda around 32-35ish degrees of timing and I'm around 37-38, as anythng higher really doesnt much from what I've seen. Getting 30-32 mpgs is the norm for me if I go on long trips.
You don't want to mess with the MAF data for this. You can watch other parameters and stuff. LTFT could let you know of a vacuum leak, STFT just switch too damn fast to bother with unless the numbers are huge but then the LTFT would follow and show a reason.
