|489whp on a mustang dyno is more than 495whp on a non load dynojet.
non load dyno will not have the load needed to hit your street target boost.
SZ is conservative in their tune.
your turbos deserve a full 3" turbo back exhaust.
somewhere between your turbo outlet and your throttle bodies you have a restriction.
I've seen that power out of cars on 93 octane and lucas octane booster with 19º advance and full 2.5" intercooler setup.