I kinda agree with 427ZO6 here. Very few people are going to do the piston soak. If this is the test ...then it tells very little on how FP performs as it is typically used.
There are a number of combustion chamber cleaning techniques and agents that are quite effective without doing a piston soak. That is, this will prove more about the technique than it will the product.
The same could be said for intake valve deposits. The agents, all of various compositions and concentrations, hit the back side of the hot intake valve to do their tasks.
The main differences in effectiveness, in my experience, is how much or how long you use a given product.
FP is sold as a perpetual additive ..being in use constantly.
This is how it should be tested.
Your test will show if a piston soak is an effective technique.