How much boost can a Stage 3 motor from Rx-7 world handle? It has wild port and polish and 3mm apex seals.

You should be asking how much power, boost is irrelevant. It depends on mainly how good your tune is and supporting mods.

elliot is right, boost is irrelevant because you could have a pea shooter running 30psi and its only making 240 whp but 30psi on a monster T4 is like 700whp. so you should be asking how much horsepower which is debatable, The normal weak points when running over 500whp are the front and rear irons cracking/ E-shaft twisting ect. right now im running around 500whp on a bone stock internal motor and if the tune stays in check it should last quite a while. A stock motor can handle amazing power levels, 90% of the time what kills a motor is an external component failing causing a lean condition.