Not sure about the effects of changing opamps on the Nuforce, but I have heard changes of opamps made to a tripath based amp and to CD players.
My 2 cents is that it changes the sound characteristics but it may not necessarily be better for everyone. With the tripath based amp opamp mod , the owner wanted more warmth in the sound and swapped opamps until he found one which seem to provide that effect, but in doing so, I thought that the amp lost a bit of bass "tightness" and "airiness". But he was willing to sacrifice those attributes to get the warmth....so I guess some opamp mods may just be the tailoring of listening preferences (assuming the key measurements aren't impacted).
Is the opamp on the Nuforce soldered into the board or chip socketed? If it is socketed, then it would be quite easy to swap around various Opamps to see the effects.