The Grizz is a decently fast machine... mine isn't, though. For as long as I've had it, it's been pretty slow, thanks to the big wheels/tires. I remember racing a totally stock Grizzly once and he beat me pretty bad. I figure the tires do a lot, plus the weight of my metal skidplates, winch, cargo boxes, etc. The clutching helps though.
It's worth noting that the Poo 800 is 170 lbs heavier than a Grizzly 660 stock for stock... so even if the Sportsman makes 60 hp l ike I think it does (compared to 35-45 hp for the Grizz depending on mods), the Grizz will still keep up pretty good if it's not being held back by mods like mine.
600 lbs / 45 hp (K&N filter, pipe, and jetting) = 13.33 lbs per hp...
770 lbs / 60 hp (stock) = 12.83 lbs per hp... which is really pretty doggone close.
I've been thinking about doing a 686 kit on my Grizz with a cam... that would probably get it as fast as the Sportsman, but I don't want to sacrifice any reliability or fuel economy. That's one thing the Sportsman beats the Grizzly in for sure -- MPG, no doubt thanks to EFI.
Rob