These sort of cars have a near vertical depreciation curve in their early years and then tend to bottom out in the mid £20k's once they're 4-5 years old.
I don't honestly think that W212 E63 residuals will be hit significantly by the W213 for the next 18 months or so as that's when used examples will start to appear and even then W213 prices are likely to be pretty strong. In my experience, someone in the market for a £25k - £35k used high-performance car is unlikely to be seriously considering an £80k+ new or nearly new car.
Something else to remember is that there's a pretty big margin between trade-in and retail prices for cars like the E63 as there's a good chance that a dealer will have to spend a significant sum on it (even if it's "only" tyres and brakes they can be looking at £2k - £3k), and warranties are expensive too. This means that buying one from a dealer and getting out of it again in less than 18-months to 2 years is going to hurt badly.