ok, i've been looking in to this myself as a v35 is next on the cards for me.
the rotors are huge, at 355mm, but from all the information i've read about them they weigh an absolute tonne. the blokes that have done the conversion have said that the increase in unsprung weight is huge and that you are best off buying 2 piece rotors from the start
here's a couple of pics of the g37 akebonos in comparison to r34 gtr brembos