Removing any cat will free up your exhaust flow and give some gains. I always run with the primary cat in and delete the secondary with a Megan Racing test pipe, this particular one is resonated so it helps tone the exhaust a bit lower. I noticed a definite difference just from this alone but of course nothing super amazing.
Most of the downpipes out there are 3 inch and going from 2.5 -> 3 -> 2.5 inch pipes will work. But in my experience the gasket that came with mine were barely a good fit I had trouble with exhaust leaks. Had to hunt down a 2.75" gasket to use instead.
If you decide to get rid of both cats then I hope you do not mind smelling fuel out of your exhaust all day, to me it becomes obnoxious and overwhelming -- not what I would want for a daily driven car. Plus the CEL light.