I only encounter very minor hiccups, everything is extremely playable. I can play all of my favorite games at 1.5x IR, which at the moment includes Mario Kart Wii, Zelda Twilight Princess, NBA JAM, and Grand Slam Tennis.
I can't speak to the Iris Pro, but I've been using the 13' (with Iris) on Windows 8.1 successfully for a bit now.