Zanzibar Buck-Buck McFate said:
Sorry guys, but I don't believe that for a second.
National contracts aren't going to be impacted by what team he plays for. They're based on the cachet he already carries. But, even then, the money there is relatively limited, due to the size of the Canadian market, and the impact Stamkos can reasonably be expected to have on market share. Using a celebrity to endorse your product is really only effective if that celebrity can be seen as having any authority about said product. For instance, Stamkos is a useful face for Powerade, but, not so much for, say, GM.
Local contracts? Well, being that I spent most of the past decade working in the advertising industry, I can assure you that the money there is not significant. Most local companies have 6 figure advertising budgets - and that has to cover production, buying ad space/ad time, etc. Bigger local companies may have budgets of a couple million, but, that still doesn't provide them with a ton of money to pay out in endorsements.
Also, there's this:
http://sportschatplace.com/nhl-picks/2014/07/28/top-10-athletes-endorsement-earnings-in-the-nhl
Stamkos is already earning a good chunk of money on endorsements (3rd in the NHL, as of July 2014). That number isn't going to be impacted significantly from joining the Leafs.