Sorry, you’re right, I should have researched, as I noted I was quoting from memory. What I mostly remembered was having read the aft tank caused problems, especially at takeoff and early in mission while forming up. The problem, which I think you’ll agree, is the plane was unstable with the aft CG, which caused control issues, and people spun in if they got distracted. My original source was I think Bud Anderson’s book on the 357th fighter group, “To Fly and Fight”, and maybe some others. But here’s a supporting discussion on the affect of the tank and the changes made to the P-51H along the lines of how it affected CG and correspondingly increased the safety of the plane (scroll down a bit).
No, but, what I meant was trimming will help the pilot overcome CG issues as the fuel burns, or to control the plane in general. For instance, when a plane is set up with CG at the outer reaches, I see it as important to trim the plane properly to help avoid overcontrolling the plane as you’ll be less likely to give input which could push you over. And, trim will change as fuel burns and the CG moves (if the CG moves as fuel burns). I realize I wasn’t clear on that point. (I often wonder about the people who complain about how the plane reacts in different modes of flight, especially at takeoff, where their trim is at? Granted, it won’t necessarily change the performance of the plane, but it will help make takeoffs, etc, smoother.).
And, then, though this has nothing to do with being aware of CG, but, since we’re talking about a WWII fighter, and he asked about what it was like to be in the heat of battle, I remember being surprised by accounts from pilots who talked about how much time they spent spinning trim wheels while maneuvering…
The real point is what I said at the start… Yes, they were aware of their CG and made changes to it appropriately. I should have left the rest out. 
As pennance, from the Pilot’s Handbook, Section 10, pg 3…
Control
In extreme cases, a CG location that is beyond the forward limit may result in nose heaviness, making it difficult or impossible to flare for landing. Manufacturers purposely place the forward CG limit as far rearward as possible to aid pilots in avoiding damage when landing. In addition to decreased static and dynamic longitudinal stability, other undesirable effects caused by a CG location aft of the allowable range may include extreme control difficulty, violent stall characteristics, and very light control forces which make it easy to overstress an aircraft inadvertently.
A restricted forward CG limit is also specified to assure that sufficient elevator/control deflection is available at minimum airspeed. When structural limitations do not limit the forward CG position, it is located at the position where full-up elevator/control deflection is required to obtain a high AOA for landing.
The aft CG limit is the most rearward position at which the CG can be located for the most critical maneuver or operation. As the CG moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or turbulence.
For some aircraft, both fore and aft CG limits may be specified to vary as gross weight changes. They may also be changed for certain operations, such as acrobatic flight, retraction of the landing gear, or the installation of special loads and devices that change the flight characteristics.
The actual location of the CG can be altered by many variable factors and is usually controlled by the pilot. Placement of baggage and cargo items determines the CG location. The assignment of seats to passengers can also be used as a means of obtaining a favorable balance. If an aircraft is tail heavy, it is only logical to place heavy passengers in forward seats. Fuel burn can also affect the CG based on the location of the fuel tanks. For example, most small aircraft carry fuel in the wings very near the CG and burning off fuel has little effect on the loaded CG.
