Leasing would be a game changer on P99.
I think Torven did some pretty robust testing of the distance per zone. Some of his findings were peculiar... like certain zones he marked down as a 200' leash range while others were 600'. And furthermore some mobs were unlimited like VP dragons while the trash in the zone leashes normally. Edit: here it is:
http://www.eqemulator.org/forums/showthread.php?t=39819
To get it correct would be a very big undertaking and you're still making some assumptions. But I think setting it to something like 600' for all zones would be an improvement over the current state.
This was only in Kunark/Velious zones per the old guides... on live all zones leash though.