Probably most likely the wheels/tyres. You need 20" rims and not the standard Dunslip tyres
I found the ride to still be a little floaty (it is a 2tonne barge) and mine didn't have adaptive suspension which firms up in the corners so I fitted £250 eibach springs, the car looks so much meaner and now handles itself pretty well for a big car, although if I was regularly going to throw it through twisties I would upgrade the brakes, just ask Will/Graham/Darren etc....
When you say you test drove a sport, was it a. XFS or an R-sport as there is a BIG difference, can't say I have ever felt like I can't push my Jaaaag, if anything I have to hold back or I would loose my license very quickly