The last time I've tried double thick ceramic magnets in a pickup (a Tar Back clone that I've built), they all exhibited different Gauss/milliTesla readings on our lab meter and the sound varied consequently with each of them.
So, IMHO, it's not possible to generalize.
I'd only bet for a stronger magnetic field, since thicker magnets are most often more charged than thinner ones.
But the effect of a stronger magnetism might have noticeably different consequences according to the coils involved...
With high inductance coils like in a X2N, the power of thick ceramic overloads the whole EQing range (flat response from bass to knee frequency / resonant frequency).
With twice less inductance like in my TB clone, the sound becomes only more treble/bass heavy and therefore apparently mid scooped, the depth of the mid scoop depending on the magnetic strenght involved. With a thinner ceramic bar, magnetism is weaker and the tone seems... thinner.
A double thick A8 should be a tad weaker magnetically than ceramic but should also increase the inductance compared to a Ceramic mag or to a normal A8, since more AlNi(Co) = more ferrous content, which is the inductive component in such mags. Now, there's less iron in A8 than in any other AlNi(Co) so the inductive change shouldn't be so pronounced.
Sorry for the long answer. FWIW. YMMV.
