Why wouldn't Jason wait to test free agency? Sure there is always that chance you suffer a major injury or have a bad year, but neither of those is very likely for Heyward. He is going to get huge bucks next offseason. If he has a similar year to his career numbers, I'd say he'd gets at least 8/200. If he has a breakout year that he is capable of, he might get Stanton money. I would be very surprised if he extends with St Louis, unless, as Ramadon said, they back up the truck for him.
As much as I like Heyward, I would not be willing to pay him 8/200. The most I would go is maybe 7/154, and that is a stretch.