FP60 will not literally add HP (as you stated before) to your motor, but it will keep your fuel system and combustion chambers clean to allow you to keep your motor at pretty much full potential all the time.
Without running FP60 (or something similar), over time deposits will form in any engine, robbing some potential power and efficiency. How much and how fast this power and efficiency is lost, depends on many, many variables.
FP60 is also said to assist with more complete combustion, which may provide better MPG and possibly a little more power, but minimal.