I do not believe the Geforce 7xx series cards can make use of the CUDA rendering because the GK chips have not been enabled to use this by Mainconcept who build the encoder in Magix. There were a few 6xx series cards that still used the GF chips from the 4xx and 5xx series cards that allow Cuda rendering, but anything newer will not work as I think the OP has found.
monjanse - As far as I know there has not been an update in MC's encoder nor is it likely there will be. While it is noticeably faster with CUDA there are quality hits in the rendered image that make it unpopular.
As John has shown, the 750 GPU will be utilised by magix but only for the parallel rendering of some effects and transitions during export, which can speed things up a little, however experience has shown that is is only marginal.
The main benefit of the GPU is during preview, for the acceleration of effects, filters and transitions.
I create a ram disk as if I use a WD 2 TB hardrive it takes 4 hours and 30 minutes for a 8 minute clip.
If I use the Raided SSD hardrives its 4 hours if I use the ram disk (made from DDR3 ram) I cut my time to 3 hours. This was before I clicked the GPU rendering. So hardrive read and write speeds matter allot. Now with what he recomended above it cuts the time to 2 hours for the same clip.
Now in Corel video studio to do the same clip it takes half the 1 hour 30 minutes on the ram disk. The big difference between corel and magix is the color correction to grade the video is like night and day. So I use both if I want speed corel is faster. If I want better editing features then I use magix.
I am not sure what to make of your tests because 4hours 30mins is an astronomically long time to render an 8 minute clip. With a system like yours even using just the CPU and no GPU acceleration it should take no longer than 3 to 4x the duration of the clip eg. 32 minutes for an 8 minute clip.
I have seen little evidence that disc read and write speeds play any part in the rendering process with modern hard disc drives unless there is some major bottleneck in the system setup. It is the processing of the effects applied and the decompressing / compressing of the frames that takes the significant amount of time.