So yeah, if you're just wanting more yield I believe you've probably maxed out the capabilities of a 3'x3'. Based on those DP Ult harvest, you're pushing 700g/m2+. I'm aiming for 500g/m2 with my perp SoG setup, 700 would be a dream.
Without more space the best way to push out more yield is probably to find the strains that can get you high g/m2 with shorter seed to weed days. Calculate with g/m2/day. IE a 500g harvest from a 70day strain is better than a 600g harvest with a 100day strain.
And +1 for mephisto.