There is an assumption that two aircraft coming off the line are identical, I can tell you they aren't.
1500 RPM in one aircraft might translate into a higher angle of attack due to reduced prop efficiency, then there are rigging issues, paint, wing attachment accuracy..
There was a time when I was contracted to fly three of the same model jets, interesting to note the cruise speed variances between the different models...such that when taking delivery of a jet I was able to determine during a prebuy that the jet I was buying beat the book by 17 knots at alt..
Not huge, but considering some jets are 15 knots slower then book, that can be a swing of say 5-8 percent...over the course of 10,000 hrs? You do the math and decide if at end of 10,000 hours, you would like to have sat in the cockpit 800 hours longer, x 200 gallons per hour...which also translates into range issues, probably higher stalling speeds and lousier handling characteristics.
Most pilots won't know the difference, but if you sit in the cockpit all day long with nothing do but look up numbers, it kinda sucks to know your in an aircraft that can't even do the book, but it's really cool if you can beat the book, get there a little faster and know that at the end of a 700 hour flying year, you didn't have to sit there for 50 hours longer then you had to, which is a week of your life.