I read a few reviews of superzoom lenses last year and out of all tested they picked up on the fact that it was slightly slower than other lenses,sorry to be a bore but as before with other lenses i went for the Sigma 80mm-400mm OS which seemed to come out on top in all of the reviews i read.
The only real difference i could see with the Nik lense was that it had better optics than others tested but it would do it costs a lot more than the other manufacturers,for the stuff i do i couldn't bring myself to justify the extra as i have always found Sigma lenses to produce brilliant images,that's why all my lenses are Sigma and iv'e just won another on Ebay....
PS: Sparrowhawk/Ostrich & Sandhill crane in wildlife gallery were shot on the Sigma 80mm-400mm OS.