Good questions. Regarding airing down, since I'm still running the OEM 20" wheels, there's not a lot of sidewall to allow for much of that, so no, I don't air down. It's always preferable, of course, to have 18" wheels (or better yet, 17"ers like our Wrangler) to get more traction aired down but, since the Passport is driven mostly on-road, I'm just sticking (for now...) with the OEMs. When I do go off-road with the Passport, I typically take the easier trails and am careful to pick a line that avoids sharper hazards. On difficult-rated trails I (we) take the Wrangler.
On the question of tire size difference, the 2020 EX-L (and Sport, I believe) came with 245/50 R20 tires, so stepping up to the 255/50 R20 tires just gives a bit more width and wheel protection. It doesn't impact, at least not materially, the speedometer reading. The Touring and Elite packages have the 265/45 R20 tires standard, or at least that was the case with the 2020 model year.