Any time you transplant it is important to saturate the new container, the goal being to merge the two soil regions. This will of course put a lot of water into that new container, and as you say, it should take some time for the plant to use that much water. The goal of transplanting is to make a larger plant, and indeed it is normal to see a very strong growth spurt shortly upon moving her to a larger space as the roots expand out to find the new limits to the environment.
Knowing that the roots will soon catch up, you can cheat a little bit on the first waterings after a large transplant. I did this exact thing in my current LSD log... I transplanted into 5 and 7 gallon containers from 3 quart containers. Knowing that it would take a week or longer to use all the water after the transplant and merging, I had to make sure to keep my original rootball teaming with living microlife... I could not let it totally dry out. 4 days into the transplant (bottom still wet) I watered with 1/2 gallon of water, spread out on the surface of the original rootball. 1/2 gallon wasn't enough to soak all the way down to the bottom and raise the water table, but it was enough to wet the top and the original rootball area. This was perfect, because then 4 days after that, the bottom had dried out... it took 8 day total on the 5 gallon containers and 9 on the 7 gallon... with your 15 gallon container I would figure on up to 2 weeks to totally dry out the first time, and then shave a few days off of that each wet/dry cycle for a couple of rounds until the system stabilizes at some number for the rest of the grow. While you are waiting for the roots to establish and assume a stabilized wet/dry cycle number, I would continue to water every 4-6 days with a smaller amount on the top, while waiting for the bottom to dry out for the big watering. Once your roots establish, or the wet/dry cycle gets to a point that the top could survive the period, then go back to the watering the entire container to saturation and then waiting for it to dry completely.