we used an ACL head shim on the A14 turbo we had (stock bottom end). the shim is fitted to the block with sealant (Hylomar spray can recommended) and the head gasket goes above that as normal.

I don't think you will have any problems with a conventional head gasket even at 1bar, IF the tune is right. detonation can blow the fire rings. Who cares when a head gasket is $20 and takes 30min to replace?

the E15 turbo pistons are a good idea, but ACL no longer make them. I looked into that a few years ago. So you would have to buy an E15T and use secondhand pistons (hopefully they'll be in useable condition). But if you did that, you're probably better off using the whole E15 motor...

From my experience, I doubt a stock cast piston is ever going to live to a grand old age in a turbo A series with any sort of power. But the A series is cheap and easy to rebuild... Forgies would be the go, but it comes down to the budget, and you'll get many sets of cast pistons for the price of a forged set! rather than being too worried about finding a hyperutectic cast piston, I would be looking for one that has drilled oil return holes rather than a long groove, the thickest headland, and the strongest casting around the gudgeon pin. But the drilled oil returns are the most important of those factors.

we used water injection on the A14 turbo early on in our tuning to help control detonation, and it definitely had big benefits. They can be simple and cheap to setup if you are not too squeamish to inject the water before the turbo. on your fmic setup, that won't work. It would be much more cost effective to setup a water spray for the IC in your case, and that can reap good benefits too.

I wasn't going to even mention getting rid of the twins and going the single route - I'm pretty sure you already know my opinion on that