If you are hitting 250F, something in your current system is not operating correctly. Your solution isn't a bigger capacity radiator. It's fixing your current one. *THEN* if you are still having problems because you are operating under extreme conditions (i.e. summer trackdays at Las Vegas Motor Speedway), consider upgrading. Your possible problems include a thermostat that isn't opening fully, air in the system, fan that never comes on, etc. First step I would take is to change/flush coolant. Make sure you fill with the correct stuff (or water wetter is good if not going to see any deep freeze temps) and make sure to mix to recommended ratio. Consider chaning thermostat if problem persists. It's a relatively inexpensive part so it's often worth taking a stab in the dark on that one. Lastly inspect everything having to do with the radiator. Make sure you have no leaks, no blockages, and generally clean stuff. A normally functional radiator should keep temps down around 200 when moving, even in 100F weather. Will likely creep up to ~225 when stuck in traffic, but should be able to keep temps in check when the fans come on. Make sure when sitting still that the fans come on somewhere between 215 and 230. A functional cooling system should NEVER hit 250, and if yours hits 250, I'd be shutting her off to avoid possible damage.
i have the same problem with my 996 , it never got over halfway unless i was in stop and go traffic and i live in the south , i have took the therm completely out and mine still over heats now i finally have got the problem down just got to fix it , its the head gasket, mine got so bad i had water in my exhaust , havnt got in the oil yet , so i would check to see if its the head gasket to. good luck