Thanks Lew, that sounds right - as far as I can make out the 25 mm GAU-8/A ammo did use the same projectiles as the GAU-7, and fired them at the same muzzle velocity. The gun was also modified to fire at up to 6,000 rpm, so in performance terms it was an exact match for the GAU-7. The downsides were that the modified GAU-8/A gun and ammunition were bigger and heavier than the GAU-7, but the upside was that it was a low-risk, low-cost development using entirely conventional technology, thereby avoiding all of the (ultimately fatal) problems of the telescoped-combustible-case GAU-7.
It seems that the 25 mm GAU-8/A was not taken any further because of concerns that the US Army was about to adopt a new vehicle cannon firing the 25 x 137 Oerlikon (which became the M242) which would lead to the Oerlikon round becoming a NATO standard, and that introducing two new rounds in the same calibre would be unpopular. The project leaders noted that if the 25 mm Oerlikon was adopted, the GAU-8/A would not make a good basis for a gun to fire it as it was too big and heavy. Eventually, of course, the GAU-12/U was introduced in 25 x 137, but for some reason was never adopted by the USAF as a 20 mm M61 replacement - only by the USMC in the AV-8B (plus very limited use in an AA system).
Reviewing the history of weapon development in general, I think that it would be very sensible, when proposing some advanced and untried technology, to develop a conventional equivalent alongside it, then test the two side by side to see which is best. So many advanced projects have collapsed into failure with the requirement left unmet because there was no backup.