My car had less than 5 miles on it. Even if it had 50, I'd still try to follow the break-in guide as closely as possible.
Wasn't 4 or 5k rpms the recommended limit during break-in? That's still a lot of leeway for having some fun.
Realistically though, driving off the lot like you stole it should not cause any long term issues (in my humble opinion).
As a side note: I'm not touching the fluid in the rear diff unless there's a problem or recommended by the dealer / GM. Same with the manual tranny. My last car had 165K hard miles (mazdaspeed 3) on the original clutch and tranny, and to this day it shifts just as smoothly as day 1. Same fluid.