The last I checked, ZFS had no open ENOSPC bugs. The trick that it uses is to reserve small amount of space. I forget if it is 1.6% or 3.3%, but whichever that is, it combined with other tricks is considered to be enough.

ENOSPC is a very different condition from lower performance. If you tested filesystem me at 90% full, you should find that all of them have lower performance than when they were empty. You might also find performance varies based on how you filled them.