The 2015+ certainly had a much faster engine, but it lost the torquey, punchy characteristics of the old motor. If you're not racing, or even if you are but aren't worried about being less competitive down the straight, the 2013-2014 models are worth a look too. Much cheaper, and in many ways more fun. I have a stock 09 as a spare bike that I just love riding!!

Valve springs were greatly improved in 2013. Pre-2013 bikes were hit-and-miss when it came to reliability. Some were bulletproof, and others couldn't do more than 6 to 10 days on track before dropping a valve. Except for valve springs, nothing else really goes wrong. If you get an earlier model, think about replacing the valve springs with the latest spec ones for reliability.