Factory figures quote the altezza at 210bhp from the factory for MT, 200bhp for AT. Who told you you only get 255bhp of of a Blitz supercharged Altezza? Mr Singh had his up to 310bhp on standard internals.
Supercharger on its own should see you up to 280bhp easily, then add larger intercooler, uprated fuel pump, larger injectors, stand alone ecu. Since yours is an auto ,there isn't as many tuning parts available. Brash boy in Japan do an Apexi Power FC for the autos