i currently run a standard 1200gx engine, was thinking if a a15 bottom end with biiger pistons, knife edge crank, 1200 flywheel with the gx head and cam would be a good route to follow?

The A15 short block will give you an instant torque 27% increase. Huge. Phenomenal. And you'll make exactly the same HP with the GX induction setup.

But why do bigger pistons? That's a miniscule improvement at a large price. Knife edge crank is only needed above 8500 RPM.

Don't forget: with an A15 you'll need a heavier gearbox and differential.

you said i will get exactly the same hp with the gx setup? isn't the gx head better, i looked at tha a14 & 15 head and my ports and valves is bigger than both. i'm planning on running it with the standard twin su's but i do have a proper ported and polished manifold with twin dhl40 carbs, don't know what's going to be best since i drive the car every day

Torque is related to how much air (and of course fuel) the engine can ingest in one revolution, so a bigger engine = more torque, all else being equal. HP is roughly connected to how much air (and of course fuel) you can breath per minute (or per second, or per hour) - so if you use the same basic head/carb setup you'll be limited by how much air they can flow, so the larger engine might make the same power as the smaller engine with the same 'breathing' (it will of course make that peak power at a lower rpm than the a12).

If you set the dellortos up to give the proper a/f curve across the entire rpm/driving style spectrum, they will be damn hard to beat, and potentially will support more airflow than the gx setup would. The current gx induction setup will run on an a15 but it probably won't have the right a/f curve above about 2/3 throttle and above about 4000-4500rpm (it'll probably be richer than it needs to be, but that's purely a guess on my part)

there's absolutely nothing wrong with dellortos or webers on a daily driver, plenty of alfas and fiats ran them on what were fairly modest output engines (no offence to alfa/fiat lovers) They don't 'only' have to be for race efforts (though they are clearly the kings of circuit race carburettion) and the right emulsion and idle jet setup (in particular) can return decent fuel economy when driven accordingly, and good power when driven harder.

The main thing to focus on is they are only viable if you get them dialled in for the combination, which will require either dyno time, or trial and error logging via a wideband ego sensor on the road. Believe it or not, the second option is the best, it'll get better driveability than practically anyone will achieve setting up on a dyno. it also means there's a lot of air moving through the radiator as the car picks up speed, so less chance of overheating vs multiple dyno pulls.

The GX head has much, much larger intake ports than an A14/A15 head.

Although expensive Im almost convinced that in the name of fuel economy in a powerful A series, a knife edged crank and lightened reciprocating parts is
worth the investment if you are keeping the engine for life.
A friend of mine has a toyota 7K with many modifications and swears
by the knife edged crank, lighter pistons with lightweight aftermarket rods
and billet flywheel. Its quite an expense to recover but he says the response
and better fuel economy are worth it.

The GX head has the "best" shaped combustion chambers right from the get go. So On a N/A motor with all the basic mods, you're going to achieve better results than you might with the H89. (A15 head)
Do bear in mind however, that the A12GX head only has the smaller A12 valves. However the seats can be opened up to take the 37 and 32mm A14/A15 valves, without the (Considerable!) expense of having larger valve inserts installed into your head. As Dave already said, the GX head has the biggest inlet ports of all the A series heads. And whilst it's not uncommon to increase the port size on a lesser head to the same extent, by starting with the better unit any improvemements made to it will leave you better off than you would have otherwise been. (Dollar for dollar)

Even just fitting that gx head will give you more power due to a increas in your compresion ratio. Better fuel economy due to the gx head combustion chamber has the perfect quench area. Its like the best of both worlds!

What makes you think the GX combustion chamber is smaller(less cc) than standard A12 "heart shaped chamber" head?

It doesn't. But he's talking about putting it onto an A15. With the better chamber design of the GX head, the combustion should be more stable. So the engine should be able to stand more timing. (all things being equal) I CC'd my GX head, and I think from memory that it was 29 or 30cc's. I've never done an H89, so I couldn't compare.