My only two concerns are his impact on draft position over the next three seasons(which I think are crucial) and his cap hit.
Stamkos is the calibre of player you could spend multiple years tanking for, and not get. Edmonton for example tanked for years, and it wasnt until this past year with McDavid that they got a Stamkos calibre prospect. Signing Stamkos guarantees us a Stamkos calibre player. Tanking for another season, or another two full seasons, or whatever, could just end up with us walking away with ~Schenn/Kadri.
Also, signing Stamkos is adding a free elite asset up front, where we are already deep. It would allow us to for example trade JVR for a young top 4 RHD. What would that do? It would upgrade us both up front (because Stamkos>JVR) and on the blueline (because a JVR calibre trade acquired dman would obviously be a solid addition).
Then on top of that, the state of the farm right now is fantastic. We're CURRENTLY considered a top 3-5 farm system, and we're about to add another top ~3-5 pick and likely ~14-15 picks overall.
As long as the contract is decent value, I think you add Stamkos. It upgrades our forward group, gives you the flexibility to make a trade to upgrade your blueline, and with a farm system bursting with talent, including very legitimate bluechip prospects like Marner, Nylander, and our 2016 1st, you could reasonably project the team growing into a very good team with the ability to sustain long-term success within a relatively short period of time.