im not sure bout this 'shim'......is it a metal plate? if so it will need to have a gasket surface on each side, making it a headgasket anyways
lowering the comp with thick headgaskets is pretty old school nowdays anyways. It all comes down to the tune, as to how reliable the engine will be.
you will be fine running 8 psi on stock engine and gasket IF you have a decent ECU AND tune to match. detonation and ignition tune would have to be the biggest killers of atmo to turbo conversion.
IMO i would run the turbo on the engine you have with an aftermarket ecu.........get some GZE injectors and have a GOOD tune done.
otherwise go down the GZE engine setup and dont have to worry bout comp ratio and pistons......ls1 is 10:1, you can run 10psi on them ALL day. tune, injectors and fuel pump and you got 300kw at the wheels