the easiest way to lose weight is to go on a diet, then remove all the things you don't need in it like the back seat(really... who uses their skyline's back seat for anything other than shopping bags) spare tyre and boot trim. Then replace heavy parts with lighter composite materials, start with the cheap easy ones to get, bonnet, fenders, boot, then move on to doors if you need to. DON'T fit a wanky fibreglass bodykit to your car, as it'll only weigh you down... nissan's wind tunnel tested their factory aero kits to be quite aerodynamic.
More power won't make your car slow down quicker when going into turns, lose weight before you add power.