[...]
I believe they are all within a percent or so of 50/50 weight distribution. Being shorter the V8 sits a bit further back...
I have no real experience of any I6 E39 but my 4.6 V8 feels very well balanced and nimble and has accurate and communicative steering; if the I6s are an improvement they are starting from a very high baseline.
Some must be the motor, but some is down to the gearbox, brakes, diff(?) and perhaps extra standard equipment...
...I guess. :)
A