In all honesty, for a easy as they are to kill, the drop tables are already too good.
The thing is that the task is so rare to get and extremely expensive to extend, so when you do finally end up getting the assignment you're getting 60 +/- kills just to end up getting one or two decent items, while the rest are steel/iron items or herbs. it's just not rewarding at all for an 85 slayer level exclusive task.
For example, you get sand burrowers at level 60 and can leave there with 6 rune scimitar, 5 rune maces, 5 r2h and an obsidian amulet, while at lava/rune golems which are 85 slayer you'll be lucky to leave with a rune kite shield and a full inventory of herbs meanwhile iron/steel items litter the ground.
[img]http://i63.tinypic.com/20t5wrl.png[/img]