The Axis 800 will out perform the Viper in all ways. The 2S will use more fuel (and oil) and needs premium, but for me, those are small prices to pay for superior performance. What stopped me from buying the Polaris was the need to rebuild every 1500 miles. The short rod 800 loads the cylinders walls too much on the power stroke and your compression will be down 20 lbs at the 1500 mile mark. It can ridden probably to 4000 miles before failure, but if you are putting on a lot of miles, then this could be a rebuild each year. And certainly, you will be down power most of the season.
A few companies make a kit to raise the cylinders and use longer pistons, but I didn't want to take this path.
My Viper is a fun and fast ride. It took a few rides and a little tuning to really wake it up. It still won't quite hang with a well ridden Axis, but the difference is small enough to be invisible most of the time on the trail.
Jim from Canton