I did it to my 650 a long time ago. I instantly liked the change. The biggest thing to me was a more open sound, sorry for the generic description. I let two other Head-Fiers listen and they both agreed that it was an improvement. They both had 650 also...one had me mod his on the spot.
i did it to my vintage HD600 some time ago, it was a notable difference, more detail, some of the veil was gone, also phones became little more bright (a little more treble), i asume it is a must have for HD650 wich are more dark than HD600