A catback doesn't net you that much power. It will though help with reaching full potential of the perrin map reflash. Are you sure it is 285whp or just 285hp (at the crank)?
Does the car have the stock exhaust on it or is it an aftermarket system. Kinda having trouble following which pipes you are referring to. Sounds like the 3rd cat pipe and intermediate pipe (contains resonator) are th epipes you are having issues with.
Are you talking about running stright off the downpipe? Possible, but something i would not recommend.