I've had both in the same guitar.
The A8 JB seems to be clear and even. I would compare it to a very hot and balanced Jazz bridge. It doesn't have a lot in common with the standard JB tone. For people who have a JB but don't particularly like the tone but still want high output, I'd recommend swapping in an A8 magnet instead of a ceramic. It's a pickup that (like the Custom 8) has tons of output, but it has the balanced EQ of a lower output pickup.
The Custom 8 is, strangely enough, like a JB on steroids. It's very full and warm, but with a slightly bright top end. It's aggressive and chunky, but still very clear. It does the JB singing lead tone pretty well, but manages to be meatier at the same time. Power chords sound massive, and leads cut right through. I'd highly recommend it for metal and hard rock.