I don't know the G's but I do off ramp tests.
My Z4 35i with 19's performance tires hits 68 mph.
My 550i with 19 in. performance tires hit 66 mph.
With all season 19's it hit 62 mph.

My X5 M Sport with 19 in performance tires hits 64 mph before any squealing. Impressive in my book. I bet the 20s with the extra widt would be darn close to the Z4.

I just realized, I've got a G meter app for my ITouch. I'll have to do a test!!!
But I'm out of town for a week. I'll report back....

__________________
Richard
Sir Snaps-a-lot, 9 Time Dragon Slayer (54 runs!)
2011 X5 50i Alpine White, Biege Nappa /Lt Poplar M Sport
2009 Z4 35i, Black Sapphire/Ivory-Black/Anthracite

Gone but not forgotten: '08 550i, '06 X3, '06 650i, '02 M5, '99 540i