I'm goona say between 10-15.
Maybe a tad more on a FI setup.
There are several considerations here...
1. Local conditions:
a. air temp
b. humidity
c. engine temps (are you heat soaked?)
2. Your tune
3. Your mods.
But generally speaking, more air in, more air out = usually making power, and air the very least, you get better efficiency.
Now, I imagine that at some point we are going to overrun are intakes ability to provide air/fuel to the combustion chamber, as well as the heads ability to flow all the new found air.
I am going to buy one... and since I have access to a dyno...
First going to make 3 dyno pulls and average that output.
Then going to install it, and make another 3 pulls, and average those.
Finally, as soon as I can come up with the other $750, and time to travel to Los Angeles (its mostly the time!!!...but I have been spending alot here... and need to slow up...) and have Dan do a custom tune for me for my my particular setup.