About the only use for the air pumps that come with most of these lasers is to add air bubbles to your fish tank. I have my lasers plumbed into my shop air and can adjust the flow for any application.
Table level is very important for a bed of that size otherwise your focus is all over the place. You should see very little if any attenuation from on corner to the next if the system is properly aligned and focused.
Another issue usually overlooked is the smoke/debris plume. You do not want the plume to just float up into the beam path, any particles in the beam path will reduce power. I have added fans and such to force the plume back and down to the exhaust so it does not interfere with the beam path.
Milltronics VM16 cent6, Clausing 1500 lathe with a Milltronics cent7, 80W Rabbit SE12090 laser