That's the question James Reeves set out to answer in an amusingly snobbish-sounding video. He fired four different types of ammunition through a low-end DPMS AR-15, and a high-end Bravo Company model. The results deflated his ego somewhat - although not altogether.
That underscores an important point. When a design has matured to the point of general availability and adoption, one can assume that most of the examples out there run reasonably well - otherwise nobody would be buying them. High-end models, made of superior materials and manufactured with stricter tolerances, are likely to be better than el cheapo equivalents over the long term; but the low end is likely to provide adequate basic services to those who don't need a race car or a purpose-built specialist weapon.
I figured this out in the early 2000's, when I began working with AR-15's and equivalent weapons to help my disabled shooting students. It wasn't too hard to buy component parts and put them together much more cheaply than the finished product was selling for in gun stores. Firearms manufacturers responded to the growing home-build movement by reducing their own prices. Over the past few years that reached the point where $500 factory-manufactured AR-15's were freely available, making the economics of home-built weapons almost irrelevant.
Now, of course, given the sudden shortage of firearms and ammunition following the catastrophic events of the past year, that's changed yet again. Parts I could buy for $50-$100 in the first half of this year now sell for $300-$400, if and when one can find them (which is seldom). Even the cheapest AR-15's are now in the high hundreds of dollars, and high-end guns are double or triple that. As for ammunition, prices are through the roof and still climbing. I doubt the situation will get back to "normal" (or what we used to regard as normal) for at least a year, maybe two or three. I used to buy M193 or M855 US-production military 5.56x45mm ball ammunition for 25c-35c per round. Today it's often found in the dollar-per-round range (if and when it's available), and often it's more than that.
We also have the threat of restrictions and/or outright bans on the sale and possession of AR-15's and equivalent weapons. I doubt very much whether most Americans will obey such restrictive laws and regulations, but it'll make legally buying and selling them much more difficult.
Hmmm . . . sounds like the perfect time for me to sell a couple more AR-15's, and a thousand rounds of ammo to go with them. Given my health problems over the past year that have kept me from writing, and thereby generating income the usual way, that would help tide us over for another couple of months. Thank heavens I stockpiled ammo when it was cheap and plentiful!