Sending general ships back to Pearl:
0.5in Brownings M2 MG -> 20mm Oerlikon AA Gun
1.1in Mk 1 AA Gun -> 40mm Bofors AA Gun
If the ship already has either the 20mm or 40mm, the quantity is doubled. If it is doubled on the first trip back because the gun type is already a 20mm or 40mm, that particular gun does not change on the second trip back. (ie: it only doubles once)
CV's are custom coded, coming back with both gun upgrades AND more guns.
I tracked a DD (Phelps) through 2 upgrades:
AA value of 220 -> 250 -> 500

I tracked a CA (Portland) through 2 upgrades:
AA value of 932 -> 1112 -> 1432

I tracked a CV (Lexington)
AA value of 2078 -> 2208 -> 1948

Strangely enough, the Lexington comes back WORSE on the second upgrade, not better, loosing a lot of 40mm Bofors for a few 20mm's being added.
The moral of the story seems to be send everything BUT your carriers back for refits which is rather strange ... It could be that the Lexington happens to be a bug.
Obviously stocking a TF with 8 destroyers with 500 AA vs 220 AA (4000 vs 1760) is quite a difference in AA defense. Makes one think hard about just automatically sending everything back twice as the USA player right from the very start, since you will get them all back anyways coupled with you having plenty of time to outwait Japan ...