Definately do NOT get the factory gauge. It's cheap looking and doesn't work very well at all. 14.7 psi=1 bar. The WRX can take 16 psi or more esily BUT it doesn't net you any more power due to the ECU pulling timing back. Now it varies car to car. Some can take 16.8 PSI and others only 15.2 before detonating so I suggest you leave it at 15 PSI or so to start if you don't tune it on the dyno. Instead of reflashing the ECU I'd go with a UTEC as it has a lot more safety features built in and can be retuned by you if need be later on. Much better investment and cheaper in the long run if you plan on other mods.