The wisdom of running ET16 wheels on Sierra ET38 hubs suggests that the front tyres will scrub out earlier than they should. Is any one actually running such an arrangement and if so is this born out in practice and is there any other peculiarities that this arrangement produces

Can't imagine that you'll have a problem. The reason that I say this is that Procomp's LA Gold uses Sierra hubs and looking at the wheels that they've fitted, most of them would appear to be ET16 or ET 18 fitment. For example this:
[img]/upload/darryls new LA.jpg[/img]
Their cars are supposed to be some ofthe best handling 7-type cars around.

You may find that the Sierra hub problem of no self centring will disappear due to an increased scrub radius. Let us all know how you get on