That's essentially my setup. It's great, go for it. I posted a video on the main 'centre silencer removal' thread. If you can't find it, PM me and I'll send you a link.
Having a Remap after fitting a Cat-Back will help you get the most from your new system but you don't NEED a Remap after a Cat-Back. It's strange that you feel the car is now less responsive, but maybe it'll settle down with a few miles on it. Check that there's no leaks along the exhaust that could be causing you a problem...