The pre-cat is soley to reduce cold start emissions, which is a requirement in some countries (NOT UK).
It's not designed to catch carbon like a DPF (as petrol engines don't make soot like diesel), it's so close to the turbo so it heats up quickly to do its job within 30secs of engine start (ecu dumps fuel into combustion chamber to heat both cats up quickly from cold).
I noticed much better turbo spool and a marginal mpg increase over the 18months I kept the car for. It's worth remembering that anyone with a Milltek, BCS (or any aftermarket downpipe) will NOT have a pre-cat either.
For a "free" mod, it's definetly a win win