The axle/housing/gears are all unsprung weight. All you're doing is "robbing" power from the engine to move/turn the additional weight, and it would increase your E.T.s some. In a drag car, I'd keep the lightest axle that was up to the task. With the I-6, the Mopar 8 3/4 should be plenty. Even with a V-8, an AMC20 with one piece axle upgrade would be fine, a 9" would be overkill.
Wiki on unsprung weight: http://en.wikipedia.org/wiki/Unsprung_mass
Bob
tufcj