If you are considering a MB AMG vehicle then sharp handling and feedback probably aren't super high on your list (no offense if this is inaccurate). The 550i won't be quite as fast as the MB AMG straight line but the handle will probably be on par. It's a great car that's pretty balanced overall.
I would keep the M5 over the CLS 55. While the torque is great, I prefer NA power, and find the high-revving V10 way more exciting. Also, I'd skip the S6 or F10 550i, as I'm sure both will be underwhelming coming from an M5 (both have less power and conventional automatics, which ain't so exciting in my book).
I would say that instead of upgrading to an 08, if you 06 has been reliable, I would spend the cash you had for upgrading on a Dinan Stroker motor for your 06. Now that would be exciting