Yeah, well I've got a W floribunda "Harlequin", definitely grafted, growing in a good spot, bought from very reputable source (Wisley), and I prune it hard twice a year, reading the RHS instructions religiously. It grows like bxxxxxy, and it has never flowered and it's about 6 years now. Presumably you will say they propagated off one that wasn't anywhere near flowering yet. And I have a W sinensis albiflora, in a ridiculous place, very dry, fighting next door's leylandii, it barely grows at all, never pruned it, bought it from the local sharks, and it was already flowering in the pot, and continued flowering every year, it is barely 4ft tall. My mum has some kind of a blue Wisteria in a ridiculously small pot, probably only three or four litres, a couple of years old, and it is 8 feet tall and growing much better than my albiflora, and covered in flowers. It isn't watered regularly because they hvae another house they go to. They were going to take it to the other house to plant it, which is why it is still in that small pot, but they haven't got around to it.

Which all goes to show you just can't tell. The beautiful ones covered in flowers I see on the local cottages generally look like they have been there for decades.