Not much. Apparently since you HAD a cat you either have a Calif. model 280 or a ZX? You didn't say. About the only emissions related item you could remove (not legally however!) would be the EGR system, however that really won't gain you anything except a cleaner engine compartment. Virtually everything in the fuel injection system is emissions related, and all emissions related items are needed for the f/i to operate correctly. Rather than trying to eliminate items, why not just clean the engine compartment really well, replace hoses with new ones (color coordinated and silicone perhaps), new color coordinated plug wires, shine up the metal parts and clear coat them, clean up and bundle the wire harnesses nicely with color coordinated ty-wraps or perhaps wrap the bundles in plastic split loom. Paint the radiator and any of the accessory mounts, and perhaps get in and even paint the engine block and shine up the aluminum parts with aluminum polish? A new shiney coil and dizzy cap, perhaps a strut bar, and a cleaned/painted air filter box etc.and the car looks like a winner!! Don't concentrate on removing things that will only cause you headaches and troubles down the road either trying to make the car run, or pass emissions testing in the future (when the tree-huggers pass a new required testing bill) or trying to sell the car to someone who WILL have to try and pass emissions. You want more performance? Try a header and intake to start along with a complete performance tuneup and making sure that everything you have already is at factory specs.
My engine bay looks fairly clean in my 280zx, but one thing i have power nothing, my belts only power my waterpump and alternator, thats it, and i'm very happy with that, just a basic car, so it has more power at te wheels lol. Lots of room without AC and all that other crap.