2' x 2.5' floor space

I have a cab, with 2' x 2.5' of usable floor space. So far I've been growing one plant topped once for 4 with LED and I've been getting 2 - 3 zips a run. I am looking to increase my yield, an was wondering if I should go with multiple plants or if I should grow one plant with longer veg and more training? Would love for some of the communities input.

400w hps in a cooltube
scrog
Sog
More plants = less veg = same yield
Less or 1 plant = more veg = same yield
Big rootzones = more nutrient uptake
Thick stems = more nutrient movement
Co2 speeds up plants and increases yield

But more than anything the single most important factor is ENVIRONMENT, have it all dialed in so everything is in optimum range.

Just a few ideas for you, hope this helps