Sounds a little extreme i have 1" thunderbolt drop springs on my front and they didn't have to change camber at all. Im about to do the 2/3 drop on mine with the 2 degree bolts from summit i can't see one more inch throwing it out so far that it can't be corrected. Sounds like your alignment shop wants it to be dead on spec which will probably never happen with lowered/lifted application. id ask to see the machine printout as long as its not red on that angle with proper rotation you should never notice it. its only when they are extreme negative or positive that it will eat tires. So being a little out of spec should be fine mine is not dead on and i rotate every 6k and my tires show no signs of uneven wear.
New DD-2009 Ford Mustang GT/CS candy apple red.
(sold)2003 XLT Tan 3.0L V6 5-speed, Lots of mods.