Saw the Q&A after our back and forth.
I never thought Stormbreaker was more powerful than the IG. Just that Thanos didn't know how to react to it so he didn't use the IG the proper way. Here's the thing though. His pre-stone power level is not specifically addressed in the film. Their movie, their Q&A I get it. We have to go with it for now.
However, Gunn specifically showed the power ups after possession of the stone in GotG even if people were not specifically using the stone (just by having the stone Ronan went from say power level 50 to power level 100 even when he wasn't specifically using the stone) , the Russo's didn't specifically show or not show it within the film. But it does leave the possibility open for someone else to make a movie and show Thanos doesn't have that strength level, or is more evenly matched for Hulk without the stones. A4 or the prelude comic may address this and I may be totally wrong, or A4 may kill Thanos and it all becomes irrelevant. We just haven't seen enough of Thanos without the stones to fully understand his power level yet.