XSL for merging huge set of input xml strings

Gurus,

We have a case where we need to merge multiple input XML strings, each
of which has a header and multiple items,  into one single big target
document that contains the header within the first XML string, and all
the items in all the input xml strings. The target willl not have any
"xml strings".
The input is pretty huge - lets say around 50-100 K at the least - and
the number of input xml  strings could be anywhere from 1 to 600.
The xsl would need to parse the string into xml, and then merge them together.

input

<messages>
  <message>... contains xml stirng with <header>.....
</header><item>....</item><item>....</item>....</message>
  <message>... contains xml stirng with <header>.....
</header><item>....</item><item>....</item>....</message>
 <message>... contains xml stirng with <header>.....
</header><item>....</item><item>....</item>....</message>
  ......
</messages>


output

<target>
   <header>
          ...
   </headeR>
   <item>
     ...
   <item>

     ....
</target>

In this case, the xslt costs should be pretty sizeable - am wondering
whats the best option to take ... is xslt the preferred option still ?
Will doing the transformation in java make it any better? What about
any other options?

Please advise.

Ram

--
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-Ramkumar Menon
A typical Macroprocessor