I don't think 15 lbs extra are that big of a deal when you are listed as being 6'6", 265 lbs, and an elite athlete. 4 weeks later, he's probably back to 265 lbs.
Hopefully, you're right and lots of conditioning coaches are wrong. If he spends 2/3 of the season in mediocre play like Lattimore did last season, the Saints' pass rush will suffer.