wont need forgies to run 15psi but the headgasket will go sooner or later... i wouldnt go runnin 15psi through the top mount interheater on a hot day... and id get a 3inch exhaust before i wound it up to 15 also... noone can ever give u an answer on how much boost your engine can take for all we know its had the tits thrashed off it in japan.. depends how its been treated. but to be safe 12 - 13 is fine even with the ceramic turbo..
but then with the t28 youll need a new exhaust manifold , most likely new water and oil lines , new dump pipe or new flange atleast.. if thats the turbo you want to build the car on do it but if your just buying it until u can afford something else id stick with the ct26