Since we cannot get a proper over-the-engine strut brace in the 550i, I've been thinking about getting the Dinan carbon fiber brace. And now that they will soon offer a rear strut brace, the combination seems intriguing.
Here's a nice review of the Dinan brace in an M5.

 

I felt the same handling improvement as mentioned in that link you posted. I will probably get the rear strut brace also if it doesn't cut into trunk space much.

ACS does make an over-the-engine strut brace in CF but it's about $1400. There's also another one I've seen on eBay for about $200-$300.