Originally Posted by Chip

It depends entirely on the application, for example a Vauxhall 2.0 16v XE engine comes from the factory with a very good map on it, if you dont change any mechanical parts at all to change the VE of the engine, then remapping it is more or less pointless, the gains will be tiny.

On a turbo car, changing the boost changes the VE, so you dont need to better the map for existing VE values in order to improve things, you just need to put a half decent value again the new VE.
Which is why even fairly clueless people can get reasonable gains on most turbo cars when mapping them!