More than six years after critics began calling for a full economic study of the Delta tunnels plan, the Brown administration released one on Tuesday, finding that the benefits outweigh the costs — albeit by a slim margin for some water users. Delta interests immediately dismissed the study as skewed and speculative. The new study looks only at the first of the two tunnels, which now are expected to be built in phases after officials couldn’t get water districts to commit to the full $17 billion cost.