> That the optimization option in Saxon EE often makes it not 
> necessary to use xsl:key 

Correct.

> but that in many situations you can 
> obtain even greater performance by fine tuning the match 
> attribute by hand using xsl:key?

I'd say "some situations". Particularly those situations where keys are
being used for Muenchian grouping, duplicates elimination etc. Even better
is to rewrite such applications to use xsl:for-each-group or
distinct-values().

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay