I have tried the nixim mod. Its made of an unusual type of laminated 'rubber?' (or some sort of synthetic equivelent). The feel of the pedal was much more like a hydraulic brake pedal and reduced the amount of pedal travel. The problem with this mod when playing GranTurismo is that the pedal travel is limited and you cant fully apply the brakes. This would be fine when playing computer sims as you can calibrate this. I cut some circles of a few different densities of soft rubber and some plastic circles out of an ice cream container to go at each end. I mixed and matched the different pieces until I got a firm pedal that I could push to the stop with a fair bit of effort. This worked quite well.
I have fitted an apelectrix load sensor in the last few days. The old sponge rubber mod was prabably easier to use with threshold braking. Pedal travel is reduced to about one third with the load sensor which is great for heal and toe if you use the clutch. At this time I have had no improvement in lap times with the sensor, and feel that it is harder to be consistent with trail braking, but this is only an early observation.
The short answer, if you dont play computer sims, the nixim rubber will prabably prevent you pushing the brake to the stop without eventually breaking something. Try making your own.