I don't have an environment similar to yours so I don't have a direct recommendation, but I will say that adding iPads into the mix will not add a huge additional load onto your JSS. They tend to be a bit more lightweight from the management side than Macs, They only inventory at most once a day (automatically) and don't usually have large software deployed to them, especially if you'll only be using App Store apps; then it becomes zero software coming from the JSS.
That said, a JSS managing ~2000 Macs running on a 3 y/o XServe 10.6.8 sounds to me like its nearing its limitations. Especially since you're also using the same box to do your imaging. Doesn't sound like that's going to scale well without some changes.
I would think in the very least you're going to want to start breaking some of those pieces out, as in, get separate Macs to handle the imaging if possible. Consider also putting your MySQL db onto a separate system (cluster) just to avoid having too much in one place.
Also, is that same XServe the distribution point? You didn't mention that. If it is, by all means considering moving your primary DP onto something else to lessen the load on that server.
Speaking frankly, are you held on using a Mac server? If not, think about moving to a Linux or Windows Server, either physical or VM, if you can. You may not have those resources though, not sure.
I'm sure some folks here have a similar situation as yours and can make more direct recommendations on the setup that has worked for them. I would also ping your account manager at JAMF on this, as that is part of their job, to help with upgrades and migration onto more robust setups. After all, it means a sale for them for up to 4500+ iOS licenses, so they have a definite interest in making sure its going to work well for you. I'm pretty sure JAMF has some documents with general recommended server specs based on the total number of managed clients (iOS and Mac) that will help.
