That makes sense to me. Super Saiyan 3 is a boost to the already established range of power. Goku's Base/SS power by this point has been established.
So, for example, this is all theory of course, but this is how I think of it.
Regular Base/SS
Starting from a point where they surpass the Saiyan limits, probably around 50,000 or so. Super Saiyan becomes an option.
Then, the power in Base/SS from Namek to the Androids is generally the top end of this range of power. They can make some moderate gains off of their Base/SS power from Namek, but they are generally at the same plane of power. This is why they need to "surpass" this level. Mere training will not allow them to break through the wall.
Grade Forms
Now the Grade Forms allow the Saiyans to unlock a whole new level of accessible power. Grade Forms are merely an inferior method to mastering the Super Saiyan. The Grade forms do allow for massive gains in power that the Regular SS could not allow for. It is another road or door that has been opened. Which leads to very vast power gains, but still, is nothing in comparison to the true, proper path. This form has it's limits as well, Vegeta most likely is already somewhere around those limits as in he is starting to experience diminishing returns. I think he can still get stronger over the years, but he's more or less back stuck on this level of power. A power completely superior to Regular Super Saiyan, but cannot be anywhere close to Mastered Super Saiyan.
Mastered Super Saiyan
This brings out a Saiyan's full potential is Base and SS. There is no comparison to the Grade Forms chain of power. I think you get the point here. So Super Saiyan 2 and Super Saiyan 3 are transformations on top of an already established range of power.
Under this theory, which I feel is implied by the story, Goku as a Super Saiyan in the ROSAT should not be vastly different than what he was against the Androids.