The GTS 450 has 192 CUDA cores instead of 336 (like the 460). Also, even though it is clocked higher, it is essentially a much slower card.
Thus it takes more CPU time to keep it fed. At least, I think that could be the issue. The GTS 450 is a good middle- to low-end CUDA crunching card but good for other, less stressful applications. Given that GeneferCUDA requires a double-precision GPU, and the GTS 450 is a lower-end and slightly less "capable" DP card, it's not that surprising that it takes a bit more CPU time (and I'd imagine more runtime in general) to complete a task.