Just to clarify, there are two different Yoga 11 Laptops. The Yoga 11 uses the nVidia Tegra 3 ARM processor which is used in many tablets and this version runs Windows RT. The more expensive version is the Yoga 11s which uses a ULV (ultra low voltage) dual core Intel Ivy Bridge CPU (i3/i5) and uses the integrated Intel HD 4000 graphics core.
The Intel HD 4000 is at best okay for games. ULV CPUs tends to have lower clocked versions of the HD 4000. It can be okay for some modern games depending on how low your expectations are.
The RAM can be upgraded, but it only has one RAM slot and the maximum supported RAM is 8GB. Additionally, it seems to must remove the keyboard to get to the RAM slot which means you need to partially disassemble the laptop to do so.
Because the is only one RAM slot, the RAM will only be running on single channel mode rather than dual channel mode. That basically means the RAM will be running at half the normal bandwidth. This can affect performance especially the Intel HD 4000's performance since it will only be working with half the bandwidth. I don't know what the performance penalty is, but 15% - 20% seems to be a reasonable assumption.
Given the performance penalty and the fact that a ULV CPU is being used. I would say at best you could run modern games with mostly low graphic settings.