..... I've also run a couple of stock models and they do great with a 16" and ok with an 18". If I were running one in stock form I'd put a 16" on it. This motor loves to rev and will spin over 15K all day long. But if it drops under 10K then it falls flat.
This year's version has a new jug and piston. They bumped the displacement to 50cc and the porting is much different. Ed tells me that it has more torque at lower rpm and doesn't rev up as high as the previous motor. I think he said it's limited to 14,100 rpm. I'll be able to tell you more after I get the saw.
....