Aft CG means that the CP/CG couple is nose up, so to maintain trim you have to have an up load, extra lift, on the tail. This gives you better range than a forward CG. Big ships nowadays usually fly with aft CGs, not aft of the safe limit, but getting near there, and accept that the static longitudinal stability will be less.
At height, the dynamic longitudinal stability will also be less, IAS for IAS, and the combination of aft CG and high altitude cruise can make the handling a touch dodgy. The modern fix is to compensate for this by adjusting the control response in the flight control system.
We had a thread on this in Tech Log a while back, and got a reference to a Boeing paper on the subject. I don't have the reference handy, being retired (at last), but ask Ken at BGS for the data. (
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Dick W