285 has support of opengl 2.1 by specs, one can support opengl 3.3 using its drivers. If it implement on software level, will it decrease perfomanse? Is there difference between using of the same functionality by core gl or by extensions?
The GeForce GTX 285 provides a full hardware support of OpenGL 3.3 + ARB_transform_feedback2.
There is usually few or no different between core or extension.
My personal advice would be to use core and extension only for extra features.