Air Conditioning - CAC and only return register in attic

This might be a strange questoin but here goes. My central air conditioner is in the attic in a closet. The only return register is also in the attic and is punched out in the wall going to the finished area. There are 2 supply ducts in the finished attic. I recently discovered manual dampers on each of the ducts so since no one uses the attic I shut one of them off. This I figured would deliver more air to the other vents which is needed since the 1st floor never gets as cool as the 2nd floor.

By doing this i'm making the temperature in the attic warmer. This warmer air is going into the only return register. Is this going to affect the cooling capacity of the unit? Will it work harder/be more expensive now that the return air is of a higher temperature and it has to cool it further? Would the air coming out of the supply ducts be warmer due to this?

Appreciate any help you can provide. If this is causing more strain/less cooling capability i'll open the damper back up. Thanks!

I'd leave the damper open. Most of the heat gain is from the attic anyway.
Plus, it increases static pressure against the indoor blower, causing higher amperage draw: more wear.
The duct sizes might be marginal to start with, due to keeping costs down - installation wise.
Worst case - lowered air flow would ice the coil, along with dirty filters, coil, blower wheel...
Remember the straw and the camel's back?
Tom Beer 4U2