Interesting that there is nothing about revs in the Mustang owners manual. My 2004 LS430 with a 4.3 liter V8 had "required" keeping less than 4K rpm during the first 1200 miles. (Yeah, I'm an old fart )
My buddies 2004 Porsche Carrera 4S "required" less than 4200 rpm for the first 2K miles.
Why wouldn't Ford suggest a rpm limit for the new Mustang? Not typical IMO.
if you are talking about the piston rings, being easy on it is not what you want to do. the higher cumbustion pressure applied to the rings during hard acceleration helps push the rings against the cylinder wall thus seating them better. from what i've researched the first 20 miles or so are critical for this. I would recomend that you drive it hard at varying engine rpm's for the first 30 miles and then drive it regularly at varying rpms for the next 2000 miles as much as possible. but take it easy on the clutch and brakes till they seat after a few hundred miles. there is more than one school of thought on this subject but i am basing my ideas on my education as a trained motorcycle technician (old profession, now a communications/navigation specialist). the research i have done suports this idea of breaking in an engine but please do more research of your own. also please excuse any spelling errors as i am just coming back from the local pub.
if you are talking about the piston rings, being easy on it is not what you want to do. the higher cumbustion pressure applied to the rings during hard acceleration helps push the rings against the cylinder wall thus seating them better. from what i've researched the first 20 miles or so are critical for this.