Not to butt in... but how is the COG changed all that much by shifting weight?
It moves it towards the rear, you can compensate with taller springs etc, but the CoG will now be further back. Not by much I guess, but maybe enough to effect going up some steep obstacles. Of course it may help with coming down some
By the fuel consumption the COG changes as the weight of the gas is lessened,
I tend to always wheel with a full tank, to avoid the pickup sucking air at the wrong time. That said, yes, it does change and my vehicle will lean slightly when it's got a full tank of gas.
Given the choice I'd take a smaller tank tucked up inside the frame where the current tank is. But I wouldn't say no to a rear mounted tank either, and will be happy to see any solution available on the market place. Apart from the cost of a new tank and protection, you may also have to factor in new skid plates and mounts etc.