Besides Porcello, there are currently 3 other pitchers who will be making $20 mil in 2016. The 3 are Matt Cain, CJ Wilson, and Jered Weaver, who were all paid based on their past performance. All were given contracts that will take them into their early to mid 30s.
Matt Cain - After having an ERA 2.79 and a WAR of 3.8 in 2012, he was given a $127.5/5 yr deal. In 2013, he had an ERA of 4.00 and a WAR of 1.7. In 2014, he had an ERA of 4.18 and a WAR of 0.0. He has yet to pitch in 2015.
CJ Wilson - After having an ERA of 2.94 and a WAR of 4.7 in 2011, he was given a $77.5/5 yr deal. Granted, not as much as Porcello's but that was also 4 years ago. In 2012, 13, and 14, he had ERAs of 3.83, 3.89, and 4.51 and WARS of 2.5, 3.5, and 0.9, respectively.
Jered Weaver - After having an ERA of 2.41 and a WAR of 5.9 in 2011, he was given a $85/5 yr deal. In the following 3 years, he had ERAs of 2.81, 3.27, and 3.59 with WARs of 3.3, 2.6, and 1.8. In a very small sample this year, his ERA is 6.29 and his WAR is -0.3.
Some of those numbers are not bad, but the decline in very obvious. Out of the 4 pitchers making roughly the same amount next year, who is likely to have the best season? My money is on Porcello.