Goku did surpass SSJ with the Grade forms, and then discovered that was the wrong way to utilise Super Saiyan before trying to master it. The anime/manga spell that out pretty clearly.
Keep in mind this source also states he was able to maintain the Super Saiyan state as a major difference rather than focusing entirely on the power aspect. Moreover, the context of the statement is him surpassing his previous limit in reference to before the Rosat training, which is pretty obvious even without FPSSJ being stronger. Furthermore, even if we were to take your view based on this source, that's concerning the anime whereas the Daizenshuu focuses on the manga. That is, if not for another anime guide going against FPSSJ > Grade 3....
"Daizenshuu 10" or TV Animation Guide Part 3
Herms translation: ”Grades 2-3 are powered up versions of the Grade 1, which Goku, Vegeta, and Trunks have the ability to transform into. Full Power is a type where they are able to unconsciously exist in the Super Saiya-jin state, and even the wildness of their personality vanishes. Only Goku and Gohan can transform into this."
Absolutely no reference to power is made for FPSSJ, despite the previous sentence saying Grade 2 and 3 > Grade 1 in power.