Will the cam from an A14 or A15 fit into an A12? If so, will it make any difference performance wise?

I don't think so. The 1971-1973 A12 has the distributor under cylinder #1, the 1974-up engines have it under #3 (and it's driven by the cam).

But maybe it's possible. All the cams seem to have two drive gears on them, but maybe the other one is for the oil pump?

Don't quote me but I think they interchange. The A14/15 cam would just have an extra, unused gear in an early A12. If you're talking newer A12 then it's a sure thing as they have two gears too.

If you run an electric fuel pump, then you should be able to use a later, usually lower performance, smog cam in the early A12 & the A10 block. The second drive gear in the middle of the cam is not used in the early engine, but the fuel pump eccentric is in a diferent spot also. It would then NOT pick up the early fuel pumps arm.

If you want to go for a more performance oriented cam, then buying a new or reground cam that is tailored for your application is really the best choice.

As 2332owner has said, everything from '74 & up should interchange, but using the '74 & up A12 GX cam in an A14/15 would be a better swap, as this was the best of the stock cams.