That "vent" on top is the natural draft exhaust for flue gasses. generally speaking, it's not wise to tamper with those, and assuming the heater is sized appropriately to the warehouse, likely not big enough to produce noticeable cooling if you did.
Product - Unit Heaters - F | Reznor
Are there any windows you could open up top? an actual exhaust fan up high could be productive (3000cfm+ each). It may actually be cooler with no additional work if you turn off your air circulators - assuming they're large ceiling fans blowing the hot air down.