I could do the following easily by making a recursive template and
then looping through the contacts, and passing on the corresponding
benchmark value.  But could xsl:key make for a shorter, more elegant
solution?

Instead of performing an addition for each <contact>, could I simply
sum() them at once, somehow?

<types>
   <type value="1" benchmark1="540" />
   <type value="2" benchmark1="640" />
   <type value="3" benchmark1="740" />
</types>
<contacts>
   <contact type="1" />
   <contact type="2" />
   <contact type="3" />
   <contact type="3" />
</contacts>

<totals>
   <benchmark val='2660'  />
</totals>