This just happened in Wintergrasp. Dispersion activated, but following damage from hunter auto shot and mage arcane blast delivered normal damage without the 90% reduction. The log shows the damage rolling in after Dispersion activation.
Yes, tenacity was on the other team, but not enough to inflate the damage landing for 16k with dispersion reducing the normal hit by 90%. The unmodified strike would have been for 160,000 in that case.