Thornton bounced from 7 to 41 points in his sophomore season and Stamkos struggled for about... 40 games. He scored 50 goals in his 19 year old season (while Grigs was sent back to the QMJHL).
Not quite comparable. I think Grigorenko is the second coming of Filatov.
When thinking of Grigorenko, try not to compare him to top lottery selections and 1st overall picks. That isn't really fair in terms of the expectations of his development.
Look at the development curve of other 1st round centers. Guys two years older than him from '10 draft, such as Granlund, Schwartz and Bjugstad are all considered very good prospects with top 6 upside, and are just finding their way into the NHL now, and are still support players. That's 3+ years after being drafted.
From the '11 draft, Miller hasn't done much yet at the NHL level, Baertschi is still knocking on the door and Scheifele just made the NHL this year. Even Zibanejad and Strome are just starting to carve out a spot full-time at the NHL level, and they were lottery picks.
Less than two years after being drafted in '12, it's a bit premature to start writing Grigorenko off, unless you were writing the above guys off too. And comparing him to Stamkos and Thornton is a total straw man argument that doesn't make any sense. He wasn't even a top 10 pick, so look more at the guys drafted around them - Girgensons, Forsberg and Faksa. The challenging part for him, is he isn't eligible for the AHL like Forbserg and some others, so made a tougher decision for the Sabres.