Well, part of the problem with destroying evidence is that there are now some 12-15 whistleblowers, including some key personnel. Based on what Rep. Issa and Sen. Grassley have already shared, we know that they already have a decent set of documentation.
I believe Rep. Issa has even shown them a few documents just to emphasize that point. If anyone in DoJ starts destroying evidence to obstruct an official investigation, they better be really, really good at guessing which ones Issa and Grassley have and which ones they don't; because if they guess wrong once or twice, that is going to lead to even more awkward questions on top of the ones already being asked. And Rep. Issa has also asked for logs of the documents being refused by the DoJ, which, if the DoJ actually produces said logs, will make it difficult to disappear them at some later date.
I suppose the President could assert executive privilege over the requested documents and that would give DoJ some cover; but it might end up bringing even more media attention to an issue that I don't think President Obama wants linked to him.