Don't waste time on CAI get an SRI and they'll run with no issues. Boost Gauge is simple either basic Analogue Gauge or electronic. I HIGHLY suggest an AFR gauge. Yea it's $200 but if something it's working right it'll let you know before blowing up the engine. MPGs drop isn't much, all depending how you drive it.
05 Matrix XR 4WD and 03 Vibe GT
"If you want Government to solve your problems, then you are the problem!" BMSR
Trix MODS: SRI w/ AEM filter, Vibrant/Magnaflow Cat-back, Alutec Lightweight Crank Pulley
GT MODS: Cosmo SRI, DRL, Auto Stop