Bag seeds, unless it came from some high grade that got fertlized, will have a limited upper limit in terms of THC. Let's say you buy some decent midgrade with seeds. Maybe those seeds genetically can reach an upper THC content of 12% if raised under the ideal conditions. A high grade seed, on the other hand, raised under the exact conditions can reach an upper limit of about 20%. Your shitty "dirt weed" may only have an upper limit of 5-8%. So yes, you can improve on the bag weed if you have the ideal conditions but it won't be that high grade killer weed, unless your seeds came from a high grade plant.