A plugged cat will cause a real sluggish response, as it plugs more, the response gets worse. Easy to check... just lower the exhaust at the manifolds by loosening the bolts and just leave the pipes hang there loose, with an air gap between.Basically unhooking the exhaust.Then drive it, If performance is restored, Bada Bing , you found the trouble.
If you are getting injector codes, they are bad. Replace them.
It would be a real good Idea to perform a compression test to be sure that you mill is up to snuff as well, before dumping a bunch of cash into injectors and cats.
Fact is stranger than Fiction