Hi John,
The 1975 XJ6C that I just bought this year has an OSJI interior from the previous owner's restoration. I can tell you that they seem to be correct, at least in the case of the XJ6C, and the leather looks and feels like that of an original. Unfortunately, I don't believe that mine was professionally installed and the headrests were for some reason never put back on the seats, however it still turned out well. I plan to put the car back to its original Silver Metallic/Red leather combination and plan to buy the kit from OSJI (I think they were called G.W. Bartlett in the recent past). I know that their kits are listed as a selling point for many restored Jaguars in this area as they are located in Muncie, Indiana which is close to where I live. I have never purchased an interior kit and am apprehensive about spending that much money and being disappointed as well. But if the kit on my car is any indication of their work, I don't think you will be disappointed if you decide to use them. Good luck and hope all goes well with the restoration! Keep me posted on which company you go with, as I am open to suggestions for interiors as well.
By the way, when I have browsed ebay for rare or NLA parts, I once in a while run across an OSJI kit that never was installed for whatever reason... just a thought, but you may check for a bargain. I found the one I need in Red, but it turned out to be for a RHD Coupe... would have been a steal otherwise. You may have better luck on the "E" type.