A well versed tuner in the industry can increase line pressure and increase shift quickness (decrease shift time) when you have upgraded clutches and billet intermediate shaft.
OK ... not sure what happened.. I could have sworn I posted a reply, but don't see it... so I'm reposting ... hope it doesn't come up as a duplicate.
I've got a '16 Ecoboost Mustang Performance Package with the 6R80, it's got an upgraded WGA (Turbosmart), Intercooler (Levels), Intake/Cat-Back Exh, and E30 Tune from Tune+ .. car currently has about 52k miles on it, trans started slipping pretty bad especially on the 1-2 shift about a month ago. It used to break the back tires completely loose going from 1-2, now it slips like crazy... I know for a fact there's no way I'm making close to 600 hp, maybe 400 if I'm lucky.. based on this thread, it sounds like the stock clutches could be the culprit?