338 is a very substantial sample size, and it's a 100% sample of his Fenway PA's.

Try this Sample Size calculator, you'll find that 338 gives you a pretty high level of confidence that your sample is close to the actual performance over a much larger population.

http://www.surveysystem.com/sscalc.htm

It's not really substantial. That .301/.406 line translates to about a .311 wOBA. with 338 PA, 1 SD of wOBA is .027. 2 SD which is statistically significant is .54. That means his true wOBA could range from .257 (Alberto Gonzalez)-.365. .365 is about what David Wright, Victor Martinez, A-Rod, Ryan Howard, and Colby Rasmus hit last year. When you factor that the 338 PA is spread over 9 seasons it doesn't really mean much.