I agree with Spike on the BHP increase.....10% at best with just about any remap on a non-turbo 1.4 petrol......but a really good bespoke remap on a rolling road, with the engine characteristics being tweaked carefully, could produce a much smoother torque curve and deliver the power at a more efficient rate. Not sure if anyone has access to the software to perform "genuine" bespoke tuning for the 1.4 petrol, but it would be interesting to see what a more responsive cam, gas-flowed head and proper bespoke tuning could deliver.
However.......the cost far outways the gains unless you can perform the work yourself. The safest and cheapest route would be to "upgrade" to the 1.6 FSi......but that's another topic......!!
Cheers
Jeff