There's a 150 point spread between the Vaio Pro 13 and Haswell Acer Aspire S7 on 3DMark 11. Both are still relatively low scorers compared to full mobile Intel Core laptops, but in our tests they did score a bit better than the average Ivy Bridge ULV. I wouldn't buy an Ultrabook for gaming, though at low settings and resolutions, they'll manage playable FPS for the games you mentioned. Diablo III is well optimized for integrated graphics and you could even run at a decent resolution.
The challenge with Intel HD 5000 is the added heat and power consumption. When you're looking at two of the lightest and thinnest Ultrabooks on the market, there's less space for cooling and a big battery. The Vaio Duo 13 (relatively speaking it has more space inside) is available with the Intel HD 5000. Even with HD 4400 it scores better than the Pro 13 simply because there's more thermal headroom.