I might check out the Lenovo Yoga 2 Pro. I have the Lenovo Yoga 13, and it runs quite fine, and I've somehow managed to play Batman: Arkham City and beat it. Yoga 2 Pro is more powerful from what I can see, using Intel HD 4400 ig and 4th generation Haswell processors. That's probably around the average or even towards the top you'll get with an ultrabook, since they're more design-focused than strictly speaking a spec sheet.
I have the 13.3" Yoga 2 Pro, and it runs great for most things, although there are some compatibility issues with resolution scaling, but if you set the windows scaling to 100-150% rather than the default 200% then it's great for having a bunch of things open for productivity.
Games are much better played at quarter res (900p) or 1080p rather than 3200x1800, but still look good on the great IPS display.
Much better on a desk than a lap too, it does tend to get rather toasty.