I think the difference to the polar moment of inertia would be almost nil given the amount of mass and it location. Most of the weight is between the front wheels and there's only 70lbs or so at most. Given that you're dropping that weight 18" or more makes a difference in the amount of body roll you get. I think it would make a big difference for turning corners since that mass gives the front end more bite. For sprints you can dial in your front axle weight with a little more or a little less gas. In endurance races it's a compromise because your handling will change as you drain the tank but that's true for any lighter car.

I assume that the ATL tank is the 45 litre version? that will equate to around 100lbs or marginally less. The important aspect about Polar Moment of Inertia is that it affects the responsiveness of the vehicle. If we could locate all the mass within the wheel base and close to the geometric centre of mass then the vehicle would turn on a sixpence (dime(?)). If you had a 45 litre tank in the original position the difference in the mass over the front axle would be small but the PM of I would be substantially less. As I said, I think the increase in the roll couple would be easy to handle.
I appreciate that we are talking around the subject without having the calcs and dynamic modelling to hand. However, my starting point from a design point of view in a Bug would be to reduce the PM of I as a preference as it already suffers in that respect with the engine slung out at the rear. I am also conscious that the axle loadings can change significantly at the front due to the consumption of fuel and I would be keen to minimise the effect. Slinging the fuel tank (a big one at that) as far forward as is possible just goes against the grain.
One last thing that does occur to me is that the tank in the front well is in the most vulnerable position particularly as that area is the only useful crumple zone in a front end collision and one behind the wheel axle line is better protected. It may be that in the use that you put the car to -sprints and the like- that such a consideration is of little concern but in the forms of motorsport that I am into it is a possibility.

evilC