as a fleet manager myself, the main problem with managing a fleet of company vehicles is that this can be a very emotive area. What I mean is that he could upgrade his vehicle, if the company allows it but it may cause others on the same salary or job to have a bit of a moan.
Plus, at the end of the day upgrading a vehicle will always costs more money for both the driver and the company.
A company car is no longer a perk, it is only your work tool. He should think of it like that. A company should only supply a vehicle if the person needs one to do his/her role. If they let their staff dictate to them what cars should be given, then they need to sort the priorites out. If one of my staff asked me that question, I would that he only cared about the car, better alloys and faster speeds and not his job.
That said, companies should always provide the most comfortable and cost effective vehicles, there would be no need for a top of the range vehicle, ubless he/she was a CEO, director, chairman etc