I don't necessarily feel that a 10* F rise in temp under the change in circumstances is alarming...might even be normal.

The posts above are all noteworthy. Remember that water cools better than antifreeze does; antifreeze primarily raises the boiling point/lowers the freezing point of your coolant. It's also a waterpump lubricant.

If you reguarly drive this car at the higher speeds, here's some food for thought: The fan shroud is there to enable more efficient cooling at slower engine speeds and slower driving speeds (which is how most cars are driven). Since the shroud embraces the perimeter of the fan blades, cool air is more efficently drawn through the radiator. But at higher speeds the shroud actually blocks the ability of the air to pass through the radiator at the rate it needs to pass with the sustained high engine speed/higher engine work.

Again, I don't think a 10-degree rise is particularly alarming under the circumstances.

Paul