generally you can top, do low stress training, or instead go with an increased plant per square feet ratio but that is rather difficult with the grobo i guess. maybe you could modify it so it can carry more than 1 plant by adding some holes
other than that you can increase the amount of thc slightly by giving your plants a night cycle of 2 - 3 nights straight before harvest. that will make them produce more resin and thc. also boosts the buds.
this is a best practice, however dont expect it to make a great difference.
also you can increase your yield by adding additional phosphorus during the final flowering stage (before flushing of course), as plants have a highly increased phosphorus need. however be careful with this dont use too much, look up for some proper schemes that suit your grow situation