New technologies are on the rise continually, so the plans would need some form of free market to function properly. Also no grand plan is going to count bakeries, butchers, grocers or all the other common stuff, those must be free market or the people are going to get pissed if there is only one place in 20 km² radius to buy bread.
But an economy that is planned for bigger products like cars, weapons, raw resources, etc could be planned to function better than they are functioning in a capitalistic economy, without the need to artificially crank up the price, more products could be produced, more time could be spend into research making things better faster. Patents are shared inside the country making research even faster, exces products could be sold to foreign countries or stored for when production comes to a halt when the plans change.
5 Year plans aren't really feasible unless they are updated each year.