Interesting reading this MH . I too am thinking about going down the S3 route (GTD is up for sale right now). I was also quite shocked that GFV for the S3 was lower than the R. Looking at 3 year old S3's on autotrader you should be able to get at least 20k on a private sale. Do you know what the maximum deposit is for audi solutions as i dont want to borrow too much money at 7.4% apr.
I'm sticking with the R now. The Audi dealers on the broker sites were looking to give me £17k for the GTD come March (18 months old), I have a solid £18.5k p/x from my VW dealer. I'll still try my arm to sell it privately through ebay/pistonheads for up to £21k at the start of Feb (they're as low as £23.5k via dealerships sold on ebay right now, will probably drop to £22.5k then).
With that difference in valuations, the S3 didn't seem quite so appealing. If i'm worried about residuals with the leasers coming through, i'll try and get shot privately at 18 months old for £22k/23k and have an S3 on order in the meantime. Might be refreshing for an S3 next time, after 2 MK7s in a row.
You need to speak to a dealer for max deposit, unlike the VW website, you can't really do finance tweaks and comparisons on the Audi site with umpteen different deposit combinations.
Let me know how you get on selling your GTD, it will give me an idea of how easy mine will be to shift privately. As long as i'm at least £1500 cheaper than the nearest dealer price and willing to drop £500, I think the inconvenience of having to arrange your own finance (if buyer not cash flush) will be more than overcome with a £2k saving (that's the theory anyway).