Math on replacement rate scaling: Going up to 20 bays brings it to 15% (the same as the universal bonus from last version), which is either 6 cruisers and a destroyer or 10 destroyers worth of carriers: anywhere from 100 DP (condors) to 130 DP (cruisers + condor) to 150 DP (drovers). Or 3 astrals and a destroyer/4 legions for 160 DP. 15% better rebuild rate is pretty good!
But:
15% CR gives -5% to fighter replacement time and I think fighters inherit the CR of the carrier: better autofire accuracy, 5% more speed, 5% more damage, and 5% less damage taken. Testing just now, the piranhas on a Condor with 100% CR do cruise at 165 speed, so the fighters at the very least inherit the speed boost. For the 20 deck case... I think that might be a better deal than 15% faster replacements, and it keeps scaling beyond 20 decks (well up to 180 DP at least).
Huh, I think I've changed my mind and do agree with you about Carrier Group, but only because while the left hand skill also benefits carriers. I'm happy with 15% rebuild at 20 decks, but 15% CR is better. For fewer decks, a rebuild bonus of like 25% (12) probably takes the edge, but in that case carriers are taking some where from 50 to 75 DP, leaving the majority of the fleet as combat ships that want the 15% CR anyways. Well thats a problem.
[Edit]
Though as others have pointed out, for bigger fleets crew training is no longer giving +15% CR, its less. Which might tip the balance back in favor of carrier group. So for a small carrier-centric fleet, Carrier Group is best because of the massive bonus; for a ~180DP fleet of any kind, crew training is best because either there are lots of combat ships that want +15% CR, or lots of carriers that also want 15% CR instead of 15% replacement; but for a muc larger fleet, ~5% CR is a smaller bonus, but a non-officered carrier support group of like 6 destroyers with thunders would get 25% faster rebuilds, which is fantastic. So the balance of the skill relies heavily on total fleet size. A bit screwy tbh.