YES. If you fit a decat pipe you will notice the difference. As the cat is a particulate filter and not a true cat on a 300tdi you shouldn't have a problem with emissions for the mot. I blanked off my egr on my 300tdi and changed the pipe at the same time and found a difference in performance. I did the same to my TD5 and have seen no difference. If the egr valve is working correctly to start with you shouldn't see any improvement. If it was knackered the you will. It is also worth flushing the intercooler out while you are at it.
It's a straightforward job. I found that jacking up the chassis - not the wheels - helped to give enough clearance, along with dropping the anti-rollbar fixings on the chassis allows it to swing out of the way.
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