So I think it "might" help some, but I'd think additional window tinting or window awings would do more.
You should concentrate in minimizing any heat transfer from outside in and vise versa. Foam blocks for the vents, shades closed, in/out trafic, shady sites, etc. all stuff I'm betting are part of your daily routine and your tired of
Regardless of the unit not being prepped a "quality" shop should be able to add framing for a second AC unit. For elec I've even seen a completely separate power cord ran to hook ups just for AC. Really simplified the install.