What performance are you taking about? Graphics performance (framerates or polygons per seconds or something)? Because CUDA (or the lack of it) effectively has nothing to do with the graphics performance of a given card - except in the sense that the last two generations of GPUs from NVIDIA support CUDA and older ones do not.
CUDA is a completely separate feature which allows programmers to write and run computationally intensive non-graphics functions on the GPU. PhysX is probably the highest profile example of this, but people around here are doing stuff like computational physics, chemistry, biology, and real time image analysis on their CUDA GPUs.
It is all about using the GPU for something other than graphics.