Should we trade him now? The answer is the same as with any other player. It depends. On what the return is.
It is worth keeping in mind that a particular player's value reflects his production minus what he is paid. Once you get past the pre-free agency years, players generally get paid the expected value of their production. So no expected surplus once you get past those years. Occasionally, you see a player signed to a well below or well above market deal. The ones on below market deals (like Chris Sale) have a lot of value. The ones performing below their contract (Uggla, Wells, etc) you have to pay the other team to take.
Going beyond the basics stated above are secondary considerations. Can you trade from an area of surplus to fill an area of weakness? Specifically, with respect to Heyward you have to consider what you have in-house to replace him. Is Joey Terdoslavich for example someone that we need to make room for in the lineup?
An interesting example to consider is Heyward for Price? Does adding Price to the rotation help us more than losing Heyward from the outfield hurt us? Would the Rays also view a Heyward for Price deal in similar terms?
Whether Price is more likely to sign a below market deal than Heyward is another consideration that has to be taken into account when thinking about that hypothetical. I don't know the answer. Wren presumably has a good sense of the answer to that one.
Btw I don't view a 20M AAV for Heyward's post-arb years as an overpay. I think that would be a good deal for the team. From a budgeting perspective we can do this. After the 2015 season Justin Upton, Medlen and Uggla all hit free agency. After 2016 Kimbrel will in all likelihood be gone. So we can pay Heyward 20M/year easily. I would actually pay him a bit more for his post-free agency years if that is what it takes. To me Heyward projects as a 4-5 WAR per year player through his early 30s. The going price for that kind of output is around 25M/year. He's not like Cano and Choo whose production will be in significant decline once they hit the second half of the deals they just signed.