Noticable gain

[C.A.R.]

IMO intake is a waste of money on a HDi, they can upset sensors and the flow of a standard OEM paper filter is just as efficient!

Get the map on ~ £100
Decat pipe ~ £50 (brand new) plus fitting if required £40-60

These are the basic things you should start with, but always combine them with a good service and high quality (if not OEM) filters and fluids.

Then go from there...