The SoC used in the N900 was pretty decent, for its era.

I know for a fact that protos with better CPUs than the 1Ghz A8 in the N9 were considered for it / the N950. However, with Elop killing the platform and desperate for it to fail and not eclipse the WP offerings, that was never going to happen ... much like the original idea for the 808 Pureview to be the first Carla, 1280x720 screen, dual core Symbian product.

Jolla have no such reason to restrict the (processing) hardware. In fact, they have every reason to buy the best available.

Obviously they're simply not going to have access to the very best camera modules, lenses or screens, or at least not at prices which would allow them to make acceptable margins on the hardware.

I suspect someone like STEriccson (who've been behind in the top-end stuff for a long time now and will be keen to pick up design wins) might offer them a competitive deal on high end chips.

Anyway, besides absolute performance, the A15 dual cores at 32nm or 28nm will have vastly more advanced power management and lower power draw (i.e. longer battery life) than the relatively ancient A8 cores at 45nm or, worse still, 65nm.