I think those two stances posted on the page before show base gets stronger. Cell wouldn't have noticed that if base became static once SSJ was achieved. Because... Base Trunks would be showing the same level as before.
And the Gohan example is also high level evidence imo, Gohan felt stronger in his base state, and Shin emphasized that it'd make SSJ even more amazing, meaning stronger base = stronger SSJ. Goku just said he didn't think that was enough vs Boo, he didn't correct the logic though. When old Kai said they were wrong, it's not like he was saying Gohan didn't get stronger, only that they were fools to believe that's the mystical power up the mystical sword was meant to give.