Evaluating short extension funds (130/30)

The current issue of the FAJ has a great piece about How variation in signal quality affects performance. On the face of it, the article deals with the generic perception that any relaxation of investment restrictions (read: long only) will invariably result in better performance. This is the selling proposition of currently fashionable 130/30 funds. But it doesn't - or only in the special case of an information coefficient which is stable over time. Managers with unstable ICs will see their performance deteriorate with increasing short positions. 

This approach delineates a way to help evaluating 130/30 funds which do not have a long enough track record in the product (i.e. the majority of vendors), but a more significant one in long only mandates. If that track record were to exhibit an unstable IC in long only portfolios already, then there might be a problem in the short extension.