EGT isn't a direct proportion of cylinder combustion temperature as it's also affected by valve timing and ignition timing so it's not quite that easy.
i would think adding things such as WI which bring down combustion temps will help you get away with higher boost levels. Dropping compression ratio should also help - have a look at the following thread for info on a Russian guy running over 500BHP on 'standard' pistons.
viewtopic.php?f=1&t=4195sklopendra wrote:
stock pistons supports 500+hp for sure but with increased combustion chamber for lower CR (15cc). Also pistons was tested at 2.4 bars of boost. Now engine covered ~8500km.
after milling.
final polishing
You will want to add monitors / safeties should either mixture go weak or WI pack up if you're pushing the limits.
You could either fit forged pistons (& possibly rods) straight away, probably won't cost much more than a WI kit, or you could go for what you can get and hope you catch ringland failure in time to save the bores.
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