Single stages suck. They are noisy, and you wai forever for them to catch up. I have an IR 2 stage 7.5 horse 60 gallon, and am thinking of getting the Lowes 7.5/ 80 gallon for my other garage where I do the paint work.
Its not overkill until you hit 20 cfm @90. Up to that point , you will appreciate every single cfm! They DO count! Unless you want to spray one pass, wait, spray another pass, wait, or DA for 10 seconds, wait 30, DA for another 10, wait 30...Gets frustrating! Go big, the first time, and you won't regret it.