Ok well you should always be able to do more dps without a macro than with a macro - that is because without a macro you can precisely use each ability in the best order at all times, which is impossible with any macro. In that respect 18k from a macro if your peak non-macro dps is 21k is not surprising. But to compare you would need to see if another macro can do more dps. Have you found one?
You are absolutely correct about all of this! However, I have played WOW for many years and recognize my limitations — one of which is spatial awareness, I get lost easily. I also find it mind-boggling when I see videos of super players in this and other games working the keyboard like a virtuoso pianist. That ain’t me! I really enjoy WOW and the challenges of dungeons and raids, and devices like macros and now GS enable me to focus on my surroundings and movements without being hobbled by constantly worrying about the perfectly correct rotation. For me, doing 17K with GS is the equivalent of doing 14K without it — lol!