I'll play on the other side of the fence
If Dak is projected as a 2nd rounder I would counsel him to consider staying. First, I would talk to him about all the other running QBs who were given high projections who ultimately saw their status slide during the draft. The bottom line is that pro football does not value a running QB because they simply do not last. The average career for a NFL running back is 2.57 years while the average career for a QB is 4.44 years. Very few people can stand up to the pounding that a runner takes in the NFL and teams don't want to waste QB money on a guy that is only going to be there 2 years. This devalues the running QB in the NFL and increases the value of a pocket passer. What is really valuable though is a mobile pass first QB, someone who can avoid even the hits the pocket passers take.
Dak is good, but he is still seen as a Tim Tebow type running QB. This will ultimately hurt his draft status right now. But if he takes another year and makes improvements to his passing game then he can move the narrative more toward the mobile pass first QB. The second round is not guaranteed money and he would be a bad showing in spring away from heading back to Louisiana as just another kid who took the early shot and didn't make it. One more year under Dan's instruction and he could easily push his draft status up into the first round.
Now, if he gets a first round grade, then he needs to go, no doubt. If Dan decides to leave MSU (which I doubt will happen), he needs to go even with a 2nd round grade. If he feels like his body can't handle another SEC season, go even with the 2nd round grade and take your shot. Ultimately though, I think the comparisons to Tim Tebow aren't helping him out here and I don't think NFL teams want to live with another RGIII situation where your money is sitting on the bench or in the training room most of the season. It's going to be tough this year unless he starts putting up some gaudy passing stats.