There are quite a few ways in which that might be considered better, or at least a step in the right direction.
[0] http://static1.squarespace.com/static/56eddde762cd9413e151ac...
Look at Norway and Switzerland
Switzerland has some of the highest quality of life you can get.
They also have higher per capita gdp
http://www.wolframalpha.com/input/?i=per+capita+gdp+of+norwa...
Longer-term it might be better if the UK diversifies more, and begins to trade less with Europe, and more with booming Asian economies or countries which share its language.
Obviously in the short-term there will be a lot of noise: FUD and posturing.
[0] https://pbs.twimg.com/media/CmBqSu-WgAEFHSa.jpg:large
[1] http://www.swissinfo.ch/eng/european-economic-area_switzerla...
Both of them are part of Shengen Area, UK no. And one important thing for the leave campaign
So while the Norwegian model is a valid option - it does not save the UK a single Euro.
I did also read the Telegraph's article about Michael Burrage's book on this [0] which said something similar, however I trust their ability to report less than the ASI:
> Our free access is not free access at all. Arguing for the single
> market on the grounds that you can avoid a 3 per cent tariff
> by actually paying 7 per cent fee is mis-selling on a scale
> that dwarfs the PPI scandal.
[0] http://www.telegraph.co.uk/news/2016/06/13/not-only-can-brit...