I believe that really is a fine line between the level of modifications that one can make as opposed to taking the M5 as it was intended by BMW. For instance, my philosophy when it comes to modifications is to retain the stock appearance as much as possible but yet adding a few "personal" touches that would make my M5 "uniquely" mine. I haven't bothered with installing the Dinan suspension kit yet because after having driven the M5 for a while now I find that it already has the perfect "handling" balance. I don't know what I'll do with the parts that I bought but it won't be used anytime soon.
BMW did a great job with the M5 but like every other car out there, it's still not perfect. For example, the cupholders is a joke, the sound system isn't as good as I'd like it to be among other tiny little quirks. It's the car as a whole that far exceeds all the little bad things. There are still other quirks that some would choose to replace with aftermarket parts rather than live with it.
To each his own!
