I'm not convinced the 370 will affect s/h 350 prices - with 07/08 ZEDs not worth £20k now and the 370 being another £10k (or more realistically requiring at least a £15k wad on top of a trade-in to up-grade), the difference is too much to hurt 350 values further.
But with less imports coming in, a lull in new 350 sales in the last 6 months, and when the economy does pick up, I reckon 350 residuals will stabilize and given the car's appeal do better in holding their value compared to most other cars out there..................... I hope