My stock transmission took countless 1.8 60 foots and down to a 3.1".
A PCM tune cranks up the line pressure when WOT making it shift harder and a shift kit would make the tranny last LONGER IMO, due to the quicker shifting there is less stress on the clutches.
As for the Mexico guy...don't get a local to tune it...unless you've got someone that tunes 3800's...or they'll mess your sh1t up. Try to find someone selling a PCM on here...or if you know someone in Cali...have it shipped there from 3800 Performance. Or if someone on the boards would be nice enough to buy it for you if you paid them

Do they not ship to Mexico?