Originally Posted by RabidWombat
Not much from just a tune. You should either need to get a new UP/DP or stay stock. There's minimal performance gain for tuning on 91 CA gas. Everywhere else, the gas is better and there are performance improvements to be had from Stage 1.
From a purely legal perspective:
SPT exhaust is CARB certified. It sounds like AEM intake and probably SPT intake are as well.
Any mods beyond that are NOT legal. This includes gutting the up-pipe and getting a new DP (even if it has a cat). Generally, a stage 2 car with a catted downpipe will still pass a sniffer, but you could potentially fail a visual inspection. The risk depends on how observant/tolerant the person doing the inspection is.
CARB doesn't care about stuff behind the catalytic converter