An 8bhp increase may seem a lot and infact is but its all relative, 8bhp onto 75 is still nothing to shout about, yes it will be noticeable....for the first half hour of driving, then you'll be left with an overwhelming 'what the hell did i just waste my money one that for' kind of feeling.
Personally i cannot see any point in mapping a N/A car at all, the gains are offset by the price of the map and if you're car is not fast enough for whatever reason wether it be insurance, first car, refusing to pay expensive fuel bills then the money is better spent on making the car look good untill such times as a faster car can be afforded.
However, if you are in the fortunate position to have more money than sense then map away untill you're hearts content.