I don't have much of an educated perspective to give you, but will give you my reasoning/opinion.
I did my first track day last month on the 9'er. I'm currently signed up for 3 more in the next 6 weeks...
I plan on buying a track bike for a few reasons.
1. It was stupid frustrating to be outbraking and outcornering everyone in my group only to be passed on the straightaways (Road America is not a very naked friendly track, I'll see how I like Blackhawk farms this weekend)
2. For $2500 -$3500 I can get a nicely set up 600 that handles better and has more power than the 9'er, and if I crash the 9er I'm likely to spend a couple grand getting it back to nice again
3. I plan to keep the 9'er indefinitely, and crashing it would really wreck that idea
4. I know based on how much I liked the first track day and how competitive I am I'll probably end up doing CCS or WERA racing at some point in the near future, and the 600 class is full of all levels and much easier to be competitive, while the class the 919 would run in you'd get smoked...
So to more specifically answer your question, if you're planning on tracking the 919, and have the money, I'd get a dedicated track bike.
If you have a more race oriented bike in mind, I'd shoot for one already set up, because you can very easily spend a couple grand just on suspension on a stock bike to make it right for you.
My plan is to run the 9'er for these three more track days this year and then try to pick up a track prepped bike this fall once everyone tries to sell (unless I find a good deal before then, I'm not good with patience... )