Wow, now that is a lot of PAT
In theory, you cannot PAT more than 65K ports for there are only that number of TCP ports available (this is an approximation) per IP address.
If a 7206 is not constantly at 90%, Your choices are very limited as a 7200 series router is a very powerfull machine.
But with the numbers of PAT, i'm not surprised.
Why, may i ask, do you need so many PATs in the first place? Can you reduce the amount that you use? What about spliting the load on different routers?