I am not sure that on its own, a CoD for the majority of cars, adds significantly to the value. In the early years particularly, there were extremes. Some were denied for petty reasons and others slipped through the net. Over the years, the jury is well and truly out as to the real value of the whole CoD system. I also doubt that the average auditor is going to establish or determine the accuracy of the CoD as it stands now, just with a cursory inspection. If the auditor is a marque expert, it is highly unlikely that they are experts on all other marques.
For a genuine single seater or sports racer with history, maybe even a saloon with history, the CoD establishes a degree of provenance, but on its own, doesn't.
For a run of the mill production car such as a VW Golf, or an MGB, I doubt the CoD adds any value at all and is seen by many as no more than a flurry of red tape/paperwork and extra cost, for no real benefit. Once a cage is welded into a classic saloon or GT, the dash modified or rear seat modifed to accommodate it, it is permanently damaged and devalued anyway.
There have been many such cars offered on TradeMe and generally, they fetch a much lower price than an unmolested standard car, even though the owner has spent a thousand or two on safety equipment.
A V8 stuffed into an Escort is obviously not pure, but at least there is no finger pointing and bonnet lifting, which is not always the case with cars presented as standard or period.
At the risk of repeating myself yet again there is no level playing field in classics anyway other than maybe the BMW E30s, which is a control series, so any cars in our series with 300cc or 500cc more than declared, are handicapped according to track performance anyway. They kid no one and therefore there is virtually no finger pointing.