My experience with kits and grapes is similar to
@Johnd, not quite as extensive but similar. The only reason I could see that kit manufacturers getting better grapes/juice would be their buying power with volume and probably only on the premium kits.
From california and chile I have gotten some grapes that I am absolutely sure weren't the best fruit available. Most of that was the basic tier grapes. But from most of the premium offerings I have purchased and especially South African grapes, I can't image how the grapes could be any better. Brix as well as all other numbers good, taste was good upfront and is even better now at six months. Can't wait to taste these 2-3 years down the road.
I have no experience with any but some of the frozen musts seem to be top notch grapes.
Also I would imagine that vineyards that have excess grapes and are selling them to the home market want to keep their customers happy with the best fruit they can. While not probably the best grapes from any given year we probably get pretty good fruit most of the time.
The worst grapes probably go to juice bucket and/or medium to low end kits. Where nobody will ever see the grapes.