Spline Smoke from Line 2

Another image in the Spline Smoke series, this time testing DOF in Mantra. The original image was rendered at 2560x2560px and needed 64x64 Pixel Samples to render clean - and 4 hours on an i7 6core @4.1 Ghz...
Somehow Mantras Ray Variance Sampling feels broken to me, no matter what crazy settings I enter, I can't get a clean result out of it...