Jeremy,
The temps you noted don't seem out of line for a C12 when climbing. It will reach those values on a steep or long shallow climb. Check engine is triggered around 215. That said, there is only a 755 ft elevation difference between Denver (5280') and the Springs (6035'). I don't recollect any significant grades on that stretch of I25, but it's been a few years since I've driven it. Was the outside ambient fairly hot? That can drive temps up also. If your reported temps occurred when running flat in the 2000 RPM range, you may have a problem with the fan speed, thermostat, etc. Not sure I'd switch to the higher pressure cap. Splitting the surge tank can be ugly. Also, be sure to check your coolant level. I had a similar problem with my Marquis and found that the heater core had developed a leak that slowly depleted the amount of coolant. The engine will run at close to normal temps until you put a load on it.
Steve