that they claim less than a 1% difference for crossfire in X16-X4 vs X8-X8. Cards, chips, drivers are faster now and impose a greater load, so today's impact is likely at least a bit greater. Sorry, but that's the closest I could come - perhaps you can find more by googling and using respected review sites.
If your system is of similar age and speed to that in the article, you won't achieve a noticable difference. Personally, I wouldn't bother doing this unless I could pop my cards in cousin's board and actually see a difference.