I don't think Seb needs a quick car - I think he needs a car that is balanced to his liking, sort of like Button in a waym though with a different preference in terms of balance, and probably with a wider operating window. In that sense I personally don't think that he can outdo someone in a tricky car that needs to be driven in a specific way - or a car that has a different balance to his liking. He can probably drive around it if it weren't too bad - like how he was struggling with understeer in his Hungary and Suzuka pole laps in 2011.
Generally though I think his issues are more easily balanced through setup - ie as long as the car doesn't have any trick/funky aerodynamics (such as the sidepod tunnels of the RB8 that were reportedly stalling at certain speeds earlier in the year) he can dial it to his liking through setup.
Personally I think Vettel is able to drive around some (not all) deficiencies, which makes it tricky to judge. Mark said in 2010 for example that the 2009 Red Bull at times had a bit of lateral instability at the rear, and Seb could live with that while Mark couldn't. At the same time it was reported earlier in the year that Mark could handle high-speed understeer, Seb couldn't.
But when the car is to his liking IMO he's probably top 1 or 2 quickest guy on the grid.
Again, we obviously don't have much insight, and all we have is an outside perspective, so my take on this is probably as good as yours. All of this is very difficult to judge because some malhandling cars are not visibly struggling, and may just look slow.
To quote one of the more respected ex-members of the forum,
747heavy wrote:Please also keep in mind, that it is difficult to "see" a car understeering if the understeer is small.
A good driver will just "know" that this is the limit, and don´t drive faster.
So it (the understeer) becomes the "limiting factor", but you can´t really "see" it from the outside or from onboard videos.
The car/driver is just slow(er), then some others.
You can only "see" understeer in more extreme form, when the driver keeps trying/pushing despite the understeer, making the US worse in the process. IMO