Fascinating that you should raise this. Impact is an issue that we are grappling with accross the whole of UK academia, since it will be measured in the REF to which I refer above. Simon Tanner has, however, done some really interesting work on this issue the report of which may be found here http://www.jisc.ac.uk/media/documents/programmes/digitisation/12pagefinaldocumentbenefitssynthesis.pdf