I think you would see a very large increase in production by going to a larger pot size, fertilizing heavily and letting the pots root into the ground.
For pot size 15 gal is ideal IMO. Its about the biggest size you can go to before the pots get too heavy and cumbersome to move by yourself. I would get the heavy duty injection molded ones that have a large rim for moving them around, something like this:
For heavy fertilization you should try to get slow release urea or a similar slow release fertilizer like Osmocote.
Lastly, it really helps to let the pots root into the ground during the growing season. For pots like the ones I linked above its really easy because the roots can come out the sides - I just bury the bottom 3" and stop watering once they root themselves into the ground. At the end of the year once the leaves drop I just chop the roots and put them back into storage.
