The fire was, as you said, in the National Personnel Records Center, so mainly involved individual personnel records, not mainly operational or a/c records. Most of those, for Korea, are at the Air Force Historical Research Agency, Naval Historical Office, Marine Corps Historical Office and US National Archives. To answer the question, KORWALD is quite accurate for the info it gives, though it's not perfect. Nothing that big could possibly be perfect. My study of Korean air war has been mainly matching up Soviet, Chinese, etc accounts of air combat and claims to accounts in US records of the same combats. KORWALD is a useful resource in that regard but it's not really intended for that, but rather as aid to finding out about personnel losses, espcially POW/MIA. But the info I've found about particular combats, operations and a/c is in line with the info in KORWALD a very high % of the time. I've found some records the KORWALD compilers apparently didn't, they found or had access to some I didn't, but KORWALD is basically a very sound piece of research.
And note, when a loss is given as positively caused by something other than enemy a/c in KORWALD, it's rare that that assessment won't hold up when you look into all the other aspects of the loss, when and where it occurred, what air combats occurred the same day, what specific claims were made by the MiG's at what time and place and circumstances that day, etc. Some listings on the web use KORWALD but match many losses in KORWALD to causes other than MiG's, as confirmation of MiG claims in published sources. That's basically crap research, to put it bluntly. The air combat losses in KORWALD labelled as such are all there were, basically, plus a few disappeared a/c which match the Communist AF's claims, and again a few omissions and errors in KORWALD. When it comes to a/c damaged in air combat, KORWALD is a lot less complete, though. However, another error seen in web debates is to consider all damaged a/c listed in KORWALD as writeoff/total losses: few were in fact.
Joe