Hi Morganic,
I read your suggested link with interest.
Now consider it - carefully.
Never loose hold of the fact that this is ADVERTISING. - and clever advertising at that.
What it says about engines is true; variations in fuel quality and altitude will affect engine performance. And it would be a problem for a car with a conventional mechanical carburetion and ignition system that was set up to run with UK fuel and altitudes, take it to a high altitude and use poor fuel, and the performance would be seriously affected.
My brother in-law lives in Canada, where he fits central heating furnaces. He has to specify to the manufacture/supplier the altitude of the installation, so the correct burner set can be fitted so it provides the specified performance.
But modern cars are not like that. They have highly complex engine control units (ECU's) that monitor things like mass air flow, oxygen and knock, throttle and RPM and with these inputs the ECU can make the necessary adjustments to the engines settings to compensate for the factors mentioned. It will deliver more poor quality fuel to compensate for the lower of combustion energy the fuel has. So loss of performance due to these factors should be minimal.
Unless the manufacture is deliberately reducing the efficiency of an engine (what would the benefit of that be to a manufacturer?) the only way to get more power out is to put more fuel in.
The performance curves of the engine are set by the manufacture. A remap attempts to redraw the curves, and regardless of what the remap supplier tries to imply, you cannot increase the power of an engine under given conditions without using more fuel.
Even though VW do supply similar cars with 1.9L engines set up to produce different power outputs, whilst in part it will be a different ECU map, you do not know what other parts of the engine or the rest of the car are also different to cope with the difference in power output.
It is interesting to note that the Remap supplier has to point out that their product will affect your warranty rights.