Thanks Nick and paco.
So, if I'm getting this right -
On a nearly saturated 59°F day, I can estimate a 1% reduction in power available, piston or turbine.
On a nearly saturated 109° day, this would more likely be a 3-4% hit on power in a turbine. The piston engine was not required to be tested to anything above standard temperature, but I should be prepared for a similar effect.
These examples assume you are engine limited as RVDT pointed out (adding MAP as a limitation for the piston).
Hot air can handle more water vapor before saturation, so it makes sense that the performance will be degraded to a greater degree.
