8 inches is good for plants that do not need to go deeper - so most annuals will do well and some close to the surface growing perennials. So you aren't doing too bad to get a good harvest of potatoes. If you lack space and wish to reap a better harvest from one plant - plant a potato in a barrel. In this case you plant your potato seed near the bottom of the barrel (full sun area), and as it grows, you continue to pile in more soil, leaving the top stems exposed. Continue until you reach near the top. By harvest time upturn the potato barrel and you will have a larger harvest. You can imagine that barrel can be more than 3 feet in height.