The catalyst is the most restritive part of the exaust, you should go with a high flow design but be aware, if the goverment catches you with an aftermarket catalyst, altering your smog parts is punishable by death.

One thing i've learned is dont just concentrate on horse power. What about some big front brakes. I miss my Mach1 brakes, 13 inch rotors are awsome. Maybe a good stereo? Some custom tuning might be cool. Thing is a maxima is already putting out some big hp.