The Lightsmark OpenGL benchmark does not do too well with the open-source Mesa drivers. The performance is extremely poor with the ATI Gallium3D drivers and for all of the Gallium3D drivers in Mesa 7.11-devel there seems to be spots of poor/incorrect rendering. The Intel driver though handled Lightsmark mostly correct except for a few artifacts appearing.
The Core i5 2500K was able to perform well against the discrete ATI and NVIDIA hardware on their Gallium3D drivers in the common open-source OpenGL games for Linux that aren't too demanding and don't use shaders, etc. With Lightsmark, at 1920 x 1080 it fell into last place with the Radeon HD 4550/5450 graphics cards coming out ahead on Gallium3D.