I tend to agree with dodgeman there. You're most probably struggling with some other problem rather than the compression ratio. You should sort out your fueling and timing first before you try to fix the problem by fitting lower compression pistons.

Are you sure that the injectors are big enough? Does the fuel supply stay stable under boost? Are you not perhaps running plugs that are to hot? You should probably run plugs 2 grades colder than the standard ones for the L-series. Plugs that are to hot will cause pinging, even in NA engines. What management do you run? Can it compensate the timing and fueling when boost goes up? Static timing is not going to work in a turbo car, no matter how much you retard it.

I can't tell from the pictures if your engine has one of those hot-gas driven whizzwheel pumps, but if you do, then it's not really the compression ratio that's killing your engine.
I would wager that you could improve the situation greatly by reducing the boost a little & far more importantly, running ignition timing that is right for the engine speed & boost level.

This means BOOST RETARD. As the boost comes on, the spark needs to start retarding to a point where detonation does not occur & this is best controlled with an ECU map. Use the distributor to provide a reliable TDC signal to the ECU, then map the proper ignition timing on a dyno, & let the ECU do the thinking.

End result.
Your wallet will be thinner for a little while, but you will have more power, more engine reliability, more mileage per litre/gallon & a lot less agro.
Now that's gotta be a good thing.