i think the only good way to test is to get a standard brand new(ish) car which states it has xxxbhp take it on the rolling road and see what it makes then remap it and see after what it makes then you may see how accurate it is. although you cant always guarantee a 225bhp leon would make 225, some say theres have made more standard, some less. no point in stating what sort of power it has, untill you have driven the car and how well it puts the power down is a totally different thing than quoting figures.
if the car has been looked after, has no electrical sensors down, a good maf sensor, fresh oil, plugs and clean filter. no problems with the engine it should run good with a remap. most maps are generic and have been tryed and tested on thousands of already remapped cars and know there are no problems, normally the
problem is due to a normal
problem you would get with a standard non maped car such as coil packs, sensors going down. maybe the map puts more pressure on these components and then problems show up