My experience has been that usually a rush of first class and gold, platinum, and diamond medallions not flying in first seem to board in one huge group. Not sure where silvers fit in any more. It seems that if you fly 49,999 miles on DL, they won't do anything for you besides let you check a bag for free these days. I have never really seen any gate agent enforce the zone 1 boarding. It seems that if you're in line, they'll let anyone through. Furthermore, if general boarding has already begun, and you make it to the gate in the sky priority lane, I seem to find you will wait longer on the sky priority side than the general line as the gate agents never look up or make much of an effort to accommodate you at that point. Sky priority is a huge mess. And as we have all been silver at one point (for the most part) it's a shame that Delta feels that if you fly 49k miles you aren't worth any special treatment these days. A good chunk of American business travelers are 50k and under per year.