Upgrade to improve handling

[14thbmw]

Ok I did a little search and got frustrated, so I'm just going to ask about upgrading my handling:

I have two X5s - a 2005 4.8is that handles like it is on rails, super sure footed and lots of grip, and a 2013 5.0i that is pretty good, but by comparison mushy and having poor cornering response.

Now the 2005 has 275/40x20 up front and 315/35x20 in back, while the 2013 has 255/50x19 all around. Both are all season, the 2013 are also RFT. The 2005 has the original (well, "serviced") air suspension, the 2013 is un-modified as well and has the sport package which may not be of any real value in the suspension (I don't recall, I bought it for the seats!).

The 2005 has a firmer ride which is more fatiguing on a long drive but more fun on a short drive, like my commute (two-lane, twisty). My wife is primary driver of 2005 and I drive the new beast mostly.

I am due for tires on the 2013 but I want to get it to where it handles much better without too much ride compromise.

So the question is, what should I do? Larger rims with lower profile tires, staggered set up, upgrade sway bar, ???

Thanks!