You would most likely get a better result if you use your extra light as side-lighting. You can place it in the middle of your group of plants and if you want to, you can even rotate your plants every day (or few days) so that each side of the plant gets a chance at the side-lighting.

This does indeed increase yields, as any increased lumen output will give extra energy to the plants. I would be careful about lighting them from the bottom as the bottoms of the leaves are not designed to receive light and it can damage them if you supply too much.