Well, you can get a full leather interior on the ZR1. It will be interesting to see what it looks like in person. Personally I don't see what's wrong with the C6 based interior. It's not amazing but it's not cheap looking either.
I think most people buying a Corvette would rather have a $45k car that is fast fast fast and with an acceptable interior, than a $60k car that is fast fast with a really nice interior.
Of course the ZR1 is bigger money, but they are also trying to keep it light. I guess we'll see if buyers are turned off by the interior or really don't care about stuff like that. I suspect with 700+ hp only a pulley swap away, most buyers won't care much about the interior.
FYI on the GT, if the ZR1 comes in at $100k, the Ford GT is 50% more expensive. So it's not surprising it would have a nicer interior.