There's no doubt that John and Josh are in the business of selling expensive discs, but i wouldn't go so far as to question their integrity.

I think John's concern with a disc cover would likely be that it must form a tight seal at the outer diameter with the rim otherwise aerodynamics is compromised. However, if this is done, how can it not possibly function like any other disc?

There are minor factors such as weigh and dimples that may add up to a second or two gain. This is important to a pro TT'er looking for that last micro advantage, but to a MOP middle aged AG'er such as myself, the real world gain or the coolness factor is not worth the extra $$$$.