Hey guys, I have performed a brake pedal mod on the G25 and can highly recommend it. I didn't use the Nixim block, instead just cut a piece of medium density foam from an old automotive buffing disc I had and inserted that in the spring cylinder. The 2 main tricks are:
1) make the length of your piece shorter than the length of the cylinder, this gives the "dead zone" feel to it. &
2) the amount & density of foam (ie the cross section x length) effects the pedal pressure. If you use too much or too dense a foam your pedal will get really stiff and it's motion may end before 100% pedal is reached. If you find this happens take more material off the cross section and the stiffness will reduce.
I highly recomend this mod, if done right it makes the pedal feel so much more realistic and the increased resistance near the pedal end means you can control the brake modulation easier.
I also recomend spacing the brake pedal up about 10-15mm, which really helps heel-toe (if you're a user of the clutch!)