I'm not really sure how that would work, since it's all the same people at the same company--just different products. In order for support to seem better for PS, they'd have to intentionally make it worse for shared customers... which they have about 11 more years of experience in dealing with.
I guess I'd compare it to not expecting different service at the grocery store when I buy a cart full of groceries, compared to when I just stop by to pick up a few things.
Back when they had the $79 Strictly Business plans, people would complain that they got the same support as the smaller plans... when the only things they were paying extra for were the additional resources/features, with all else being equal.
I'd guess it's the same for PS. If your site won't run on shared, but runs on PS, that's probably where the extra money goes.
If they were to add additional strictly PS-staff for more support, I'd imagine it would affect the price.
Anyway, I don't use PS and it hasn't been offered for too long, so I'd probably expect a few bumps along the way if I tried it.
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