Yes and no
I've found Powell to be cooler in summer by several degrees. Every little bit helps when it's over 100. The wind is about the same. There is usually adequate wind velocity at some time during each day, however, it is gusty and changeable due to the canyons influencing the direction. I found the biggest problem with sailing is not wind but waves. The huge amount of motor boat traffic in the summer causes wakes that bounce off the canyon walls and mix in the middle with new wakes or old wakes and creates very choppy conditions even in the deepest water. The new low water levels aren't helping any as the shore is closer in many places now. Motor or sail, Mead and Powell are both worth taking a veiw weeks to visit and explore from the water. Take plenty of water, even in winter the air is extremely dry. Take spare props if you explore side canyons.