Removed Exhaust Manifolds to find A cat! Just 1.

[ELF]

New to these cars, so please be kind.

Fixing a N52 exhaust manifold leak, i removed manifolds to replace gaskets to find only one of the two factory manifolds has Cat-honeycomb still in it. The insides of the rear one had been drilled/disolved/hacked out, looks link not a bad job at it (but over-tightening the manifold-head bolts warped the very very thin manifold flange - which caused the leak in the first place). Still...

Do I:
1. replace the rear exhaust manifold with one from wreckers.
2. attempt to drill / hack / disolve the front manifold cat.
3. i am here now - buy some ebay special equal length manifolds for fun (more sus for rego)

How much work is it to remove error codes?
Does the ECU compensate well enough for this change and do i loose any power?
Does this make the cabin noise too loud?

Thank you for your help!.

Also to note, the rear factory cats have already been removed - so running 1/4 i think it is. Cheers