Normally I think of B-vitamins as being important for energy metabolism. B15 has not been demonstrated to be an actual vitamin but must be easy to produce and has been sold as a B-vitamer. I would not invest in it..
I would invest my efforts in other vitamins. Generally, a well balanced diet or one based on formulations used these days for production will at least come close to meeting your birds needs. If your birds are stressed then their need can increase of their ability to extract the vitamins from the diet can be compromised. Then you can compensate by providing the additives or switching to ingredients that have more available B-vitamins. Grains not ground would be a start which means avoiding scratch grains and stuff that has been sitting around a long time or exposed to too much heat. Keep feed fresh, cool and dry. In my setting I try to make so birds have near continuous access to fresh greens, especially clovers (legumes) as they are good sources of many vitamins. The pepper example I provided above was largely intended to be a source of A-vitamers although it can be a good source of B-vitamers as well. Coprophagy (poop eating) which does occur in your pens can also provide some B vitamins but not all. I free-range young birds and broody hens to get around these issues when practical. If you are committed to tight confinement then the use of supplements may be worth considering, especially if you have any doubts about the quality of feeds. Also, if your birds are challenged by heavy worm of cocci burdens their ability to extract vitamins from the diet may be compromised. The use of rooster booster type products are particularly effective in that latter situation.
