You also may wish to consider why the extra heat. Maybe you can cut
down on the heat if a lot of it is coming from windows or
underinsulated outside wall or whatever. I doubt the computers are
warming it up 12 degrees. I agree the return vent is probably
necessary, but there also may be air infiltration problems identified
that may mean hot air is coming in from outside and into the wall
cavities or from the floor (crawlspace or basement) or the temp of the
outside wall and windows on the inside is way high.

Just a thought. No sense adding a return air duct if the drywall
surface on the outside wall is 98 degrees, if you know what I mean...
blowing foam or cellulose into the uninsulated cavity or sealing it
from underneath if that's the problem will help tremendously.

One simple way to find out if heat is coming up through the floor and
into the wall cavities and heating up the drywall, which heats up the
room is to go into the crawlspace with incense stick or cigar or
something smoke producing and run it along the sill plate or rim joist
or whatever you want to call where the wall sits on the foundation, and
if it's hot down there and smoke is sucked up or blown away from the
plate, that's a problem easily addressed. If it's sucking in, it's
better than blowing out. If it's sucking in, it's sucking in the air
temperature in the crawlspace. If it's blowing out, you are sucking air
from the attic. HOT!

Simply sealing off the bottom of the cavity stops the "chimney effect"
so you don't have to seal off the attic side of the cavity.

Just a thought...

abi