15C is plenty. Though, the drain limit in terms of "C" rating depends on cell capacity. The C stands for charge so a 1000mAh cell with a 10C rating can handle 10 Amps continuous. The C rating is for continuous drain. For the intermittent loads we put on an atomizer, it's safe to add 50% to the drain limit. So, a 1000mAh 10C cell can safely handle 15 Amps intermittently.

If you want to know how low you can go in terms of C rating, it depends on the C rating ~and~ the cell capacity. For example, I use a pair of 2C cells for one of my mod designs that each have 3000mAh. Each cell has a max drain of 6 Amps and they're in parallel so that doubles it for 12 Amps. I can safely go up to 18 Amps on the whole battery with the burst loads of an atomizer. At a minimum cell voltage of 3.2V, that gives me a max load on the cells of 58 Watts. The mod will not allow a load on the cells greater than 42 Watts so I'm well within tolerance.

Oh, one other consideration with a booster is supply impedance. They don't like too much of it. The higher the drain limit of the battery, the lower the supply impedance. Though typically it's not a problem unless you're taking a cell right to the edge of what it can handle. If you don't exceed the continuous drain limit, you'll for sure keep supply impedance low enough for a booster. The 20C and 35C LiPos actually have much, much lower supply impedance than what is maximal.