When the Red Sox gave Porcello that extension, there's little doubt that they overpaid a bit on the salary, but in return they got a relatively short 4 years. It's not all about the salary.
The way I look at it, it was a 5 year contract for 95 million. He was already set to make 12.5 million for 2015. And the extension was announced before he had thrown a pitch for us in a real game.