It will hurt yield for sure, how much will depend on the strain. If going from bought seed that gets expensive, If growing from clones it is a different story. Flowering early is mainly done by those doing a SOG grow with clones where they are often put straight into flower which is what I am currently trying, some veg for 1-3 weeks though. I have seen journals of people doing 12/12 from seed and getting results but unless your seed is free this dose not make much sense to me, but people have their reasons.