Ok, so with the Wiseco pistons and 0.8 bar boost, all was well. Then it started to produce knock. I would investigate that line. First off I would check all the boost piping for cracks, leaks or perishing etc. Then I would investigate the fuel supply, pressure and regulation side of things and of course, timing, either cam or ignition.
The reason I suggest that is that it sounds as if you may have jumped the gun and gone down another route without investigating the detonation problem fully, which if still there will cause damage to any engine or system you fit anyway.
You didn't mention if the det caused any piston damage or not. Mild det on forged pistons is mostly irrelevant regards damage and the pistons should be fine, unless the det was very high and visible damage is present. Just be aware of any det chips in the piston crown and skirts, for the purposes you require, they could be polished out anyway, if present.
As for using flat top pistons and a 3mm copper gasket, I see your aim with that but it's certainly not a route I would take.