My 156 was an 04 plate facelift model.
I can't remember who it is on the forum that said this (I should, because it's one of the long-time regulars), but his opinion was that the 159 is quicker than his 156 GTa point to point because of the handling advantages. The GTa hardly has stock suspension.
The difference that the 156 simply can't overcome is the difference in the chassis stiffness. No amount of springs and dampers can fix that.
Referring to the 159 3.2? I can't say I've driven a 159 but I have driven a lot of supposedly "superior" handling cars, for example the E36 328i coupe (with LSD), E36 M3 coupe, E46 330D, E46 M3 coupe, newest-but-one 530D, SLK32 AMG, Mazda 6 MPS, Mazda RX8, and the only ones there that surpass my 3.2 are both the Mazda's, when it comes to handling on our back roads.
I don't know what standard GTA suspension is like, but I can't imagine a stock 159 3.2, which is set up to be a compromise* between ride comfort and handling (to appeal to a much wider market) is better either round a track or on British backroads than a GTA. The difference in power at the wheels, nevermind power to weight ratio, is staggering! But the 159 does have its advantages, refinement, smoothness, comfort, equiptment, quality of materials if not outright reliability, but like I have said before in my eyes the 159 is more of a 166 replacement than a 156 replacement. A very beautiful car but not a direct replacement for the 156, I think the upcoming 149 will be more similar to the 156 in that respect.
*albeit a very, very good compromise given that it won best in class in those German handling tests