I also redid my tests with the macro. This time however, I used 2 different speed bows and different amounts of haste. First, I did another test exactly the same with my other tests except for that I was spamming this macro:
/use Steady Shot
/cast !Auto Shot
1.21 attack speed, ~1.427 cast time steadies, Auto Attack/Auto Shot checked, no macro:
441 Steady Shots * 1.5 seconds for GCDs = 661.5 seconds
559 Auto Shots * 1.21 seconds for shot timer = 676.39 seconds
Difference in seconds: 14.89
1.21 attack speed, ~1.427 cast time steadies, Auto Attack/Auto Shot unchecked, no macro:
500 Steady Shots * 1.5 seconds for GCDs = 750 seconds
500 Auto Shots * 1.21 seconds for shot timer = 605 seconds
Difference in seconds: 145
1.21 attack speed, ~1.427 cast time steadies, Auto Shot/Auto Attack unchecked, using above macro:
454 Steady Shots * 1.5 seconds for GCDs = 681 seconds
546 Auto Shots * 1.21 seconds for shot timer = 660.66 seconds
Difference in seconds: 20.34
2.5 attack speed, 1.67 cast time steadies, Auto Shot/Auto Attack checked, no macro:
599 Steady Shots * 1.67 = 1000.33
401 Auto Shots * 2.5 = 1002.5
Difference in seconds: 2.17
2.5 attack speed, 1.67 cast time steadies, Auto Shot/Auto Attack unchecked, no macro:
622 Steady Shots * 1.67 = 1038.74
378 Auto Shots * 2.5 = 945
Difference in seconds: 93.74
2.5 attack speed, 1.67 cast time steadies, Auto Shot/Auto Attack unchecked, using above macro:
603 Steady Shots * 1.67 = 1007.01
397 Auto Shots * 2.5 = 992.5
Difference in seconds: 14.51
It appears that using the macro greatly helps in recovering the lost shots, but it doesn't quite match that amount with it checked.
Edit: Added the original test results.
2nd Edit: Better organization of results.