I had several punctures on my S-line, I never struggled to get replacement tyres quickly (I think a day, at most).
I also found one could drive surprisingly quickly on the spacesaver.
So I don't really see what you'd gain from squeezing a full size S-line wheel in.
If you really want a 'proper' wheel, why not go for a smaller steel wheel or alloy one off a base line model?
That's what I've done with my Alpina.
Might fit into your wheel well nicer than an S-line one too.
I got a 16" alloy, with tyre, for £42 off ebay.