JCL, I'm look at the BMW website right now and here are the weight numbers.

X6 35i 4894lbs
x6 50i 5269lbs

x5 30i 4982lbs
x5 48i 5335lbs

Are these the numbers you are looking at?

From the Autobild article the X6m is rumored to run a (nurburing?) lap time on par with the M3 E46. From test driving both the X5 and X6, the X6 hands downs handles better than the X5. I believe with the same engine, the X6M would be faster than the X5M on track. The question is by how much? I agree with LeMans, that it would be similar to the M5 and M6 comparison, but the X6 has a better handling technology so I'm assuming by a larger margin.