It seems pretty obvious by now that the case for Brexit is not a promise but an aspiration. "Things *will* be better if the UK leaves the EU," we are told, but there is no real idea how this will be achieved. But, then, how could there be? The aftermath of a Brexit vote will be long, messy, complicated, and dependent on far more than what just the UK wants, as it severs itself from the EU and tries to build a different relationship. Who knows what that relationship will be? Will the countries we deal with be anxious to keep the status quo as far as possible, would they be somehow punitive instead (or at least unwilling to budge in the way we'd like), or would they see a non-EU UK as an exciting opportunity that everyone will bend over backwards for? Who knows?
This applies both ways, of course. I think the default assumption of the various bodies that have offered their analysis is that the attitude displayed towards Brexit by other countries today, one of scepticism and disapproval, is likely to carry on beyond a "leave" vote. This seems reasonable, in which case Brexit is likely to leave the UK somewhat worse off because everyone else will be in no mood to pander to our demands. But maybe the reality of Brexit would force other countries to change their tune. The only way to find out how Brexit would actually play out is to try it.
In that sense I can't say that the OECD prediction adds much material to the case for staying. It's a given that leaving the EU is taking a risk, but I can't say I believe that any attempt to quantify that risk is really going to be accurate (or, more to the point, the analysis is probably sound but we don't really know what the starting point is going to be, making any specific numbers relatively pointless). I think this is the tone the Brexit campaign should really be taking, which is to say that I don't think they really *need* any detailed analysis to be on their side. Apart from making them hypocrites, again the point is that Brexit is about an aspiration, not a promise.
I think this is the tone that Leave campaigners really need to start emphasising, though. They are far too dismissive of just about everyone who points out the risk, as if it doesn't exist. It does. They should be far more honest about that.