there isn't much 'definative' information as they are just a generic chip, so alot depends on the condition of the car. it'd be hard to find two digifants which are identical now, due to the age of them so running back to back tests is pretty hard.
also the only real comparison can be made by getting the car on the rollers, getting a good figure, swapping chips, doing it all over again. just for the time on the rollers you'll be looking at the best part of £100 plus the price of a chip. ultimately they are all mapped to a more aggressive curve than standard chips, which are a compromise between reliabiltiy, efficiency, and power.