Alright I finally approached this slightly different.
Due to concerns of just database diving.

I edited the forum/scripts/changedisplay.inc.php and set 99 to 1000
Code
if (($PostsPer < 1) || ($PostsPer > 99))

Then edited forum/templates/default/editdisplay.tpl and added a option for Max Threads Per Page
Code
<option value="1000" {if $PostsPer eq 1000}selected="selected"{/if}>{$lang.SHOW} 1000 {$lang.MAX_POSTS}</option>

Then became member and deleted PM's 1000 at a wack.
No issues.

Then I decided to run the rebuilders for PM's
Rebuild Private Messages----- No issues

Prune Orphaned Private Messages---- Invalid mode specified

HUMM!!!

Every thing seems okay so far and I restored the two files back to virgin stock code

But what does Invalid mode specified mean?

Blue Man Group
There is no such thing as stupid questions. Just stupid answers