I have glanced over the report and it seems to me that the Daily Mail is being rather misleading. For sure this particular study suggests that the expected impact of Brexit will be lighter than the Treasury report prior to the referendum predicted -- and this is true in two separate scenarios the study looks at. But the bit about migration in particular is not a prediction but an assumption, ie the study literally plugs this in from the start. No wonder they find that met migration falls to exactly the levels they set it to.
For the rest of it, I think the study is rather better to interpret as an evaluation of two different schemes for developing forecast models of trade. They spend a lot of time discussing the "gravity model" approach, where apparently the volume of trade flow between countries is taken to be "proportional to the product of the size of their economies, and inversely proportional to some measure of the distance between them", along with a few extra assumptions related specifically to the EU. The paper rejects this approach, or at least how the Treasury applied it, and -- well, essentially that's all the study really does.
At various points in the paper's introduction and conclusion the authors make clear that they have "little or no direct information about how the UK would fare outside the EU", and that "the best we can do is construct scenarios".
I am not meaning to pour scorn on the report per se, nor do I prefer the doom-and-gloom predictions of the Treasury last year. What I am saying is that I don't think there's any major basis for regarding this particular study as somehow "more right" when it comes to the utterly unknowable future that awaits us with Brexit than any other study, or to prefer this expert analysis over another one -- except that these experts happen to have produced a prediction that broadly supports your particular agenda, I guess.
One can anyway regard the prediction as a bit doom-and-gloomy, but in a different sense. As the Mail rightly points out, the authors conclude that Brexit might provide little additional changes to the growth outlook of the UK in the coming few years. But that's because we already had a bleak growth outlook, in their modelling, which is hardly a ringing endorsement for future UK prospects either: "Things won't be any more awful under Brexit than they already were going to be" is still a bit gloomy, no?
But anyway. The economic outlook of Brexit is unknown and unknowable. That applies to "Project Fear" too, of course, but there's no point pretending that the "project sunshine" models are somehow more trustworthy just because you agree with them.