I think getting to 1ACH at 50 would be extremely hard for anything that isn't new construction. I was surprised to see mine at 5ACH when I got my blower door after I had sealed a huge chunk of stuff and I'm not sure how its possible that my house still was leaking about 1500CFM. I'm thinking its largely the flue backdraft and possibly some inaccuracy from the blower door too because we didn't find that much stuff to seal. Air sealing is key though and everything I've done has definitely helped a ton but getting to 1ACH could be an extreme chore and might not really be practical.
Regarding the temp rise, if the temp rise is too high and the ductwork is restrictive, sealing the ductwork doesn't solve the ductwork being restricted, but of course it does help with causing the heat that is paid for to go to the right place.
