From what I've learnt since I've been on Autopia I'd probably leave the UPP as the LSP on silver due the difficulty in getting real depth out of that colour. Saying that, I've got so many products now that I'm going to have to start trying different combinations just to use the products up!
CMW, Natty's, S100, P21S etc will all look great over UPP on silver, but I'd imagine you'd benefit more from making sure the prep work is imaculate.
Just my $0.02,
Ben.