I dont see Diego flopping in terms of having no impact, but perhaps in the way that he wont be some saviour some people expect him to be. I think he is a quality player, but I know the kind of flaws he has, and I'm not sure everyone who has called for his signing knows, so they may overrate him a bit, and be too quickly dissapointed with impatience when things get slow etc.
But aslong as Ranieri wants him and wants to use him right (because I sadly think Ranieri will be our coach for the forseeable future....), then there isnt much else you can ask for.
Diego is simply the kind of quality player we should get, and he comes with risks and is a gamble like any player, so thats something we have to live with.
P.S Dont believe the 30m figure FOR A SECOND. Totally overpaying.