From my experience (published first app almost 2 years ago), the sales data is preserved and repopulated in to the reports after the blackout. In few cases there has been a slight drop in the sales, once the data has been restored. In those cases the actual store has been down and not only the reports.
When the reports don't work, it often also happens that the "Best sellers" list in the store shows some random items instead of the real ones. So if in such case your app disappears from the Best sellers list then less people find it and thus less people buy it. And in such sense problems with online reports DO have impact on sales. (that's assuming that you have any apps on the Best sellers list. I do)