First off, I should compare to my ML500 and the X5 4.4 which I previously owned. The Turbo and the X5 is really a totally different car.
The major difference between the 4.6is and the 4.4 which I noticed right away to the lighten steering system. Yes it is 'much lighter' than the ML and the 4.4i. I thought they changed the rack and pinon to an recirculating ball. But the spec said is still the rack and pinon...
I was quite confuse by why BMW would make a sportier SUV with a lighten steering. In fact, after I took delivery of the car, i drove into the service department and ask them to check it for me. They told me it is normal for the 4.6 be abit lighter. I guess i should do more research on it. The downside is that you don't get much feedback, but the steering feel is still very good.
Second different is the tiptronic, the downshift and upshift is oppsite to my 4.4i.. That was quite confusing too.. Why? The downshift used to move the shifter down and the 4.6 is up. But thats just need getting used to.
Handling wise is a best point of the car. I bought the car to some twisty moutain roads around the country side. It has very little roll and the best part is... The car is actually oversteer bias!! I have been able to execute some 4 wheel drift around some hairpin, which my old 4.4 will push understeer helplessly. I still can't imagine i was driving a SUV. Power output is so responsive. Just a small tap on the throttle and the car power out the corner. The car have 210miles on it now and the sidewall of the tires starting to wear :P The suspension also tuned up abit, but the ride quality of the car is still consider comfortable, actually more comfortable than my ML. I think the 20inch wheels add some road noise... thats about it.
The engine according to what I know is that it was an Alpina design which BMW bought. I was quite skeptical at first by how little displacement they increased with such big difference in performance number. But in real life, the car felt alot more powerful. Even tho the engine is based on the 4.4, they are actually quite different. Partly I suspect is the gear ratio different. The first time you rev it up to redline (which I am not suppose to :P) I can feel the real power kick in around 4 to 6K rpm. It is a higher rev engine then the 4.4i which the power dies down around 5000rpm. To compare to the ML55, the 5.5L engine seem to have more low-end torque where as the 4.6 have abit more momentum at high rpm. I should say the ML55 engine felt explosive where the X5 4.6 felt quick.
The transmission is also faster respond. It really give you a very sporty feel when push hard.
When I was driving back home tonight in the highway, i wanted to test the top end of the car in G.B. Highway. Well, at 140mph on the speedo, the RPM is at 4000 rpm in forth gear. I can't imagine what it will be like when I unleash the full potential of the car... scary fast for a SUV.
I think the only improvement for the 4.6 is the steering feel. But other than that. I can't find any SUV I love more
Let's hope the Cayenne will not take that away from the X5...