For me it's all about the fingers actually. The more I raise my fingers while streaming (and hitting keys with bigger force), the more accurate I actually am, just like playing piano for example. On stupidly high bpm (around 210 and onwards) that becomes a little different though, because you are just wasting strength if you want to press keys with big force which is unnecessary due to smaller timing, but the fingers are still the main part of the thing, at least for me.
This could also be the reason why my streaming is so terrible in 200~220 range and is fine above and below. I am still working on it though and as the time goes by I can notice really good improvements (I just can't get the bpm straight sometimes so I end up with some 100s on longer streams).