[Ferruccio]

Since no two N54's have exactly the same horsepower stock, how does, say, a higher stock horsepower N54 get affected with the same type of tune? (Piggyback, ECU flash, etc) Does the tune become more powerful by a similar "multiplier"?

[sear]

There are too many variables that will effect individual results to get the answer that it sounds like you are looking for. You can however get a solid idea of where your car might stack up by looking at the results posted by folks with similar mods and running in similar conditions as you. There should be enough posted on this forum to get a good idea for what you can roughly expect...

A sampling of the main variables that will effect power/dyno results:

-Type of dyno (variation will still exist even dyno to dyno of the same type)
-Weather (basically density altitude) (most dynos have corrections for this, but they really only help to roughly compare as weather conditions change)
- Fuel (octane levels) (the N54 especially tuned likes Octane)
- The sum of all collective modifications
- base condition of stock parts(plugs, injectors, no carbon buildup, etc)

[rader1]

Originally Posted by Ferruccio

Since no two N54's have exactly the same horsepower stock, how does, say, a higher stock horsepower N54 get affected with the same type of tune? (Piggyback, ECU flash, etc) Does the tune become more powerful by a similar "multiplier"?

Most tune only cars see a bump of about -50whp. If your car made 250whp stock, you should expect about 300whp on the same dyno. If it is a factory freak 300whp car then you should see about 350whp on the same dyno.
But as sear has said, there's are too many variables to say anything with a huge degree of certainty as pertains to HP gains and estimates.