cats and exhaust

[Famouspork]

Hi all
I went to my mechanic for a quick exhaust check, my exhaust line is full of little holes, even the muffler itself, and I have the feeling that the cats are clogged like crazy.
Would that cause my car to accelerate poorly in general, but mostly when cold?Because when it's warmed up, it's not so bad, I can live with that.

Also, I'm thinking of having a custom exhaust made for me, and only have a single, straight through muffler at the end, would that hinder the car in any way?