Changing rear end gears and considering it "modified" is debatable depending on the car. In terms of the XF where no factory rear gear options were available then yes, I would consider a change in gears to be a modification, but if my Mustang came with 3:55's and I swapped them out for a factory option 4:11's than its still a bone stock car IMO.

The term "sports car" is a vague and mysterious moniker also. Depending on what country of origin, what era etc produces different versions of "sports cars". Typically a sports car would constitute a nimble, balanced, rear drive with enough power to weight ratio to allow a spirited drive 'feel' even if the stop watch says you're doing 0-60 in 3 days. Take the Miata for a prime example. Considered to be one of the most pure sports cars ever produced, true 50/50 balance, rear drive, 2 seats and a paltry ~135hp and yet it just "works". But there are literally hundreds of exceptions to the rule (not that there is a rule).

As a new owner of a X-fire and new to this forum, I've been surprised to see so many trying to squeeze a horse here and a horse there from a NA XF just to improve a tenth of a second at a 1/4 strip. The NA XF is a canyon carver wrapped in a stylish body. Nothing more, nothing less. YMMV.