Don't come up with crack pot theories either kid. Goku's design has almost nothing to do with this. In fact Goku's design in Movie 8 was if anything different than it was the manga entirely, having a different looking aura style (Which if nothing else is just an animation aspect of the movie)
You at best could make the argument that Goku's not a Mastered Super Saiyan but we already know that the entire purpose of an MSSJ is to have a better balance of ability, not that its stronger than normal SSJ.
In Movie 5, Goku's clearly baed on his namek incarnation, in fact the fight between SSJ Goku and Cooler even takes scenes used in the battle with Freeza and uses them with the battle with Cooler such as how Goku punches Cooler away. Yet Movie 5 Goku is obviously much stronger than Namek Goku.
So when Goku comes up here and point blank states that he's only using 50% of his power (which means jack shit I will add because please insert line of Goku doesn't do any more training anyways) pretty much leaves little doubt on where Goku would be.
You know where Goku would be? Come here I'll tell you.
*Whispers* Cell Games level.
What did Toei look at the manga, see 50% Goku and just think...yeah...full power Goku right there. The same guy who wanked Broly so much he felt he needed to put him in 3 times. These guys looked at 50% Goku and inferred...yes...that will be the full power of Goku.
Now I admit that out of all the subjects I have taken in my life, mathematics is not my strong point. However...50% doesn't mean 100%.
Movie 8 Goku=Cell Games Goku.
Natural look don't mean shit. Again...we already know Goku's full power and it never changes. this is not hard but no it is to you people and its somehow hard for you to wrap your heads around.
What does Freeza have to do with any of this? We know that in the manga Freeza only uses 50% of his power. If another movie came up in the same time and it seems Freeza's using his full power, we would easily correlate that to whatever his full power was in the manga at the time.
It doesn't matter what SSJ Goku is at the time, we'd know where Freeza is at the time. If 100% Freeza in whatever weird movie he appeared in is losing badly to SSJ Goku, then it merely means that SSJ Goku has a bigger amplification.
Becauase again, we have numbers.
10x what?
Vegeta's kick? What does that have to do with anything? You may make the argument that Vegeta's only 1st ROSAT if you wish, same with the others. But we already know Goku's been through the ROSAT on account that Gohan, who became one in the ROSAT, is a Super Saiyan.
So yeah...mathematics. 50% doesn't equal 100%. Even if at the time we don't know exactly where someone like Full Powered Perfect Cell would be, we'd sure as heck know Broly is far stronger than Cell Games Goku unless we got some line in the manga later on saying Goku actually did train through those 10 day period.
But again...we do not.
It's not hard man.