Not exactly, Goku's Kamehameha was rivalled by Vegeta's Garlick Gun. Not every Kamehameha has a greater power than the user and that's shown in many other occasions.
When I say it's portrayed, I mean it's shown that upon using the Kamehameha, Goku's initial bp reading of 416 is raised to a bp reading 924
Do you deny this instance? Or do you accept that in this instance the Kamehameha multiplied his bp, but every time after this doesn't?
And it's funny that you bring up the Vegeta's Galick Gun thing, I just read another post that had something about that in it. Anyway, that 24k vs 18k can be chalked up to Goku getting weaker as the fight progressed, coupled with his repeated use of the Kaiouken, which destroys his body, it's not unreasonable to presume that his base had gotten weaker to the point that a Kaiouken X3 only matched Vegeta.
I have always considered that statement as something weird as if Piccolo thought as if that Kamehameha has the same power as the one Saiyan Saga Goku use and that's why he consider it trash. However, I don't think that has anything to do with amplification.
There's no way it's not to do with amplification, if Cell simply gathered all the Ki in his body & fired it, it would be just as strong as if not stronger than Goku's Kamehameha against, say, C19, or at least against Freeza. That's why you consider the statement as weird, Piccolo's statement viewed through this lense is nonsensical.
However, if the Kamehameha is an amplifying technique in the Cell Saga, just as it was in the Saiyan Saga, it makes perfect sense for a Kamehameha based on the Saiyan Saga Goku to be inferior. If Goku's Kamehameha has become more multiplicative since then.
We have seen Goku launching a full power Kamehameha on Cell being directly called like that or Goku thinking that Gohan with all of his power would be able to kill SPC. In none of those instances, is stated that those attacks increased the battle power of the user. Therefore, it is what it is.
It doesn't have to be explicitly stated, it's already been established that they multiply bp.