?Could sort+ be quicker than for-each not preceding

I'm expecting this has been thought of but then couldn't find mention of it..

For large <for-each not preceding> would it be quicker to do
sort list
when current-node not equal to immediately-preceding-sibling then DO
otherwise look for the next following-sibling not equal to current-node
?

I don't know if internally Saxon already does something similar to this. Or is 
finding immediate siblings costly?

If this works, I'm wondering when it becomes worthwhile?

Regards
davidpbrown