I don't think there is much risk in Tampa dealing quality assets for EK considering that's where he wants to be - or at least it's near the top of his list. Trading for him should guarantee that he locks up assuming they're willing to pay him what he wants.
I know it's easy to say "I'll just wait until July 1st" but a lot could change by then. Maybe he falls in love with the city he's ultimately traded to and decides to stay. Maybe Ottawa decides to deal him at the deadline instead, but before the deadline comes he changes his mind and decides to stay. Maybe someone else leapfrogs Tampa on his destination ranking by the time July 1st rolls around. There are a lot of variables. IMO it just comes down to how highly you covet Karlsson. If you think he puts you over the top, you make the deal.
The only thing I disagree with is the "pay him whatever he wants." TB just doesn't do that. If it's true that Karlsson insists on being the highest paid D in the game, regardless of destination and tax situation (I don't think it's a coincidence that most of the teams rumored to be involved are No Tax states), then it becomes less likely that TB was a major player in this. I don't think we can offer him much more than 10, but our 10 is worth more than most teams', obviously.