This is actually a genuinely interesting question for Marxists, more than you likely think. Personally, I don't believe there is such a thing as a clear-cut line where Public ownership instantly becomes better, down to the exact second. You can centrally plan something from the ground up, though it may not be as stable or grow as rapidly in the beginning. However, there are massive benefits to being entirely centrally planned and publicly owned, such as the abolition of money and the replacement by which with labor vouchers or other such non-transferable tokens, as well as being able to compare the entire productive forces against each other for all industries and adjust accordingly.

The thing is, there is no such thing as "true" Socialism, and if an aspect of Marxism turns out to be wrong, then it must be adjusted and accounted for to be true to Marx himself. There will be different processes in each country and different characteristics, hence why the PRC practices Socialism with Chinese Characteristics, in anticipation of Socialism with Palestinian Characteristics, or Socialism with Canadian Characteristics.