I ran 30+ psi boost on a 6.8:1 736cc WITHOUT O ringed sleeves or heavy duty studs, just an .043" copper head gasket. There were no apparent compression related issues when I tore it down for inspection. The stock flat top pistons sat about .125" in the hole.
All the 970 and larger motors I've had were all O ringed. Some saw 30lbs boost. All had big studs and copper head gaskets. Some pistons had domes, some slightly dished.
For what it's worth, here's my unsolicited .02.
As you know, to drop the compression with big bore pistons, a flat top or dish in the crown is required. I was told back in the day that the raised edges of the turbo pistons brought the squish band back (with those heads not bore matched) and the dish promoted turbulence.
Modern high swirl chamber design is pretty flat and small, and so are the pistons. Notice how much timing can be pulled from an efficient engine.
That being said, we still use hemi chambers, flat top pistons, and LOTS of timing in our fuel car.
Back in the Formula 1 turbo days, intake and exhaust valves were surprisingly close in size.
Once the air tries to go sonic past these little valve seats, it looks like our ports cross sectional area will become the limiting mega-boost flow factor. And that big bend doesn't help much either...
