Presently struggling to persuade myself to part with a fairly large wad of cash for a replacement exhaust. But would much like the freely breathing engine that JMo and others have described resulting from fitting a decent exhaust, and DNA filter/airbox and either the Kev mod or a PCV.

How restrictive is the OE exhaust? Is there any point in doing the less expensive inlet-side mods, but leaving the standard exhaust as it is for now? Would this deliver any noticeable benefit?

Or is there some glaringly obvious reason - maybe involving the cat - that I have missed, that makes this unwise or impossible?