I am in the same boat,but my worst complain is handling while towing.Lack of the SLS on the E70 is part of the problem,but not all.Another 500-700 lbs extra weight is another.I drove a friend's '12 50i with M package and it feels very close to the 4.6is/4.8is although it has only stiffer/lowered suspension,no special stuff.If you find a complete suspension from a car like that,it would work good,but you will still need the 20" wheels.
Just get a set of 20"s and see how it will feel like.Probably much better.
Putting spacers will actually make the suspension even softer.