I also believe advances in ammunition likely played a large role in the improvements, and testing older rifles with modern ammunition is always interesting, but remember to not lose sight of the intimate relationship between ammunition and chamber/rifling configuration, something with which many here, particularly the BR crowd, continue to tweak and experiment.
Winchester knew what it was doing, having at its disposal the best of the best for research and development. I think we must assume Winchester designed and manufactured its rifles and ammunition to maximize accuracy and performance of its products, first and foremost. So, while most of the rifles remain in largely fine shooting condition today, the ammunition painstakingly designed and made to perform in those rifles is long gone, making it impossible to gauge performance against modern ammunition in modern rifles.
Looking at those 200-yard average groups, it doesn't seem to me there have been leaps and bounds of improvement in accuracy with more modern rifles and ammunition. Rifles shooting those 1.25" 200-yard groups would have been averaging less than .3125" at 50 yards...with 10-shot groups, no less. I'm pretty sure none of my rifles have ever done that well, even with the best Eley or Lapau ammunition...well, maybe one in the last 20 years.
I guess what I'm trying to say is, many in the BR community use a chamber made for a specific, currently produced ammunition. The vintage 52 chambers may or may not perform with ammunition made for modern chambers, so, who knows whether or not modern barrels and/or rifles are "better" than the vintage ones?
Food for thought,