It's a decision only you can make. The difference from the 4.8i to the Cayenne Turbo is big...4.1 vs. 6.1 seconds, which is a big deal if you're doing 0-60 sprints all day long, although in real world driving it probably isn't much different. I find that if I have the 4.8i in DS mode, I have to feather the throttle so as not to look like I'm racing away from every light...90% of the time I still leave every car behind. So...the CTT would be crazy, same with the X5M, and even the 50i which does 5.1 for the 0-60.
How fast does it need to be? I don't know. It's a lot of fun as is, but if I want a sports car experience, I'm going to fire up the 545hp in the Vantage rather than the X5. For me for now, the 4.8 is plenty as a daily driver. Maybe that will change when I get a sampling of the 50i or the new CTT. Time will tell.