Correct.. the computed % changes in engine number will be approximations but they will certainly be well in the ballpark.
Now, that does not mean that some accurate 'guarantee of performance' numbers would not eventually be needed for some reason. But if this 'thing' is so good, then you will be swimming in $$ and you can then buy your own engine dyno and pay some people to run it for you LOL
Do I believe that this 'thing' will give 35% HP and torque increase? No, but that is another matter.