Some say yes. Some say no. There's the old story of manufacturers reducing performance to reduce warranty claims. In reality they set the vehicle up for reasonable power and to meet exhaust emissions. The TD4 and SD4 are not short of power and torque. MPG is reasonable already. Personally I'm not into mapping. Some have done it. If it were my vehicle I would be more interested in spending the money on making sure the servicing is up to date first. Particularly the haldex oil and filter changed if it's unknown when it was last done. If it were mine I would change all the oils. Engine, gearbox, transfer box, rear diff and haldex.