I can't figure out from the original message what the intent of using
extracting parts is all about, I guess) I say go with the current wisdom of
the crowds and see what's in use most commonly.
I would take even one step further and say that the clef is not basically
needed at all, if it is indicated that the part belongs to the soprano voice
and that the common clef of soprano voice is well established in tradition.
In that case, the clef is just a beautiful decoration, as such useless, and
could be left out.
On the other hand, if the clef is in proper use, and it is not just a decoration,
it is better that it indicates the correct octavation. This will give some new
opportunities in octavation. I will give a few examples:
1) Changes in octavation during the piece.
One clever way of using clefs would be to show change from clef G to clef G^8 and
back in order to indicate the changes in the octavation. This look more beautiful
and more professional than using 8va lines that span over more than one staff lines.
2) Optional octavations.
If you want to indicate that the octavation may be chosen according to will,
you may use the modern clef G^"(8)" indication that leaves some freedom for
the singer. A professional soprano can sing with clef G^8, and an amateur
soprano with clef G.
3) Indicate whether the piece has to be played at the concert pitch or
it has to be transposed.
With euphonium that has both transposed or non-transposed pieces, one
may use various clefs, among others clefs F, G, and G_8. In addition,
there are some modern clef indications, namely clefs F_2, G_2, and G_9,
that can be used to indicate the tradition according to which the notes
are being written. However, I have not yet seen, for example, how clef G_2
is used in practice.