[Marcello]

"Again why would the US invest all that time and money to create this huge weapon and have it only slightly out perform a weapon like the German WWII era 28mm sPzB41 (pen 66mm at 500 meters)?"

Some points to consider
1) The sPzB41 achieved those performances by being a bleeding edge, over engineered, limited production item with several issues (barrel wear inherent to the taper bore designs etc). GAU-8 ammo was designed to be practically made and expended in immense quantities.
The drag issue associated with what is an APCR design have already been noted.
GAU-8 ammo has been around for what must be over 30 years now, so it is not exactly a recent design either.

2) The "it is WW2 level stuff" claim is not sound by itself. You could knock out an Abrams with a WW2 vintage Panzerfaust. That does not make the Abrams a poor design, does it?

3) The GAU-8 high ROF means higher HE kill against soft targets, less time spent on target and to an extent greater probability of hitting the target. Such things are not directly related to each round penetration.

4) Speaking of hitting probability I have an hard time imagining how you could hit a tank size target more than six kilometers away with a fixed gun mounted on a plane.

"Im not looking to create a "uber weapon" just something closer to the real thing."

If you had an alternate a source claiming that it was let's say 88mm at 500 meters rather than 69mm it would be something we could accept without much fuss. As it is you were asking for it being capable of extreme performances (knocking out tanks at 6000 meters and the like) based on little of substantial.