i am using 2 x 597 cfm fans and i get it too, my room is totally sealed and what i did was i used thicker white plastic for the walls for reflection and used small squares of white sticky tape to strengthen the pull and used a heavy staple gun through the tape and put hundreds of staples in, i have solid walls.
you could add horizontal 2" x 1" strips of timber on the outside on the sides, and then staple through the plastic in to this from the inside.
this would help take the serious curve from your plastic on the inside of your grow area.
and i do not have a vacuum vent.
negative pressure has no effect on the plants.
even serious pressures do not affect humans deep in water, it is the speed of the ascent that matters it can cause minute air bubbles to burst through and enter blood vessels etc to enter the bloodstream.
plants do not have a lung so there is no adverse effects on them from positive or negative pressure.
only if they were high on a mountain where the air pressure is different and the air is very thin would they suffer, humans get altitude sickness from the thin air.
you are giving your plants perfect nice outside fresh air.
