We (rsync.net) do not have any numbers, but we know what happened when we broke 85% on a zpool that contained a single vdev ... things went to shit. Luckily, we expanded that zpool with two other vdevs and it
sort of balanced things out and rescued it ... meaning, enough IO happens on the other two vdevs to make the zpool viable.
However, the effects scared us enough such that we will never let it happen again ... and that is a fairly severe economic and provisioning penalty (ie., chop off 15% of your inventory on top of the three drives per vdev you already lost for parity).