Having the head out, I took some time to measure the LET squish volumes.
It's actually less than I thought, probably because the designer expects the mixture to already be quite turbulent - the air coming from the turbo and all
650mm cube is 0.65cc, around 2.35% of the total Chamber volume.
This means that the designer has sacrificed the burning of 2.35% of the mixture, in return of better burning of the rest
With a gasket thicker by 1mm (or steel plate to that effect) the new, deformed chamber will be sacrificing 4.16% of the volume with not much squish effect
These are static figures, would be accurate at revs while cranking the engine by hand.
In real-life rpm there's a lot of inertia involved, and the stock LET squish can easily be below 1% while the 'deformed' one over 5%
If you're aiming at 300bhp, it all adds up.