Auto flowers do not need a change in lighting schedule to initiate flowering and perform best under a 18/6 light schedule, from birth right through to harvest. That is their "automatic" part of this type of cannabis, the ability to flower regardless of how much light they are getting. The 400watt for veg and 600watt for bloom is a favourite combination, and I can see no reason why it should not be a great idea for auto flowers too.
So, go back to your 18/6 lighting schedule and either continue with the 400s or go all out and upgrade to the 600s. They will certainly produce more with the 600s and I can see no negatives associated with going to the 600s at all, except for the increase in power consumption.
Only normal photo period plants require the switch to a 12/12 schedule to initiate flowering, this mimics the approaching Autumnal season, which alters the hormones within the plant that "tell" it to switch from vegetative growth to flower growth. Auto flowering plants do not need this trigger to start flowering, their flowering cycle is controlled primarily by age and not seasonal day length alteration.
Hope this helps,..... Organoman.