The problem I see with this method is:
A player will, if he loots a box and find crappy loot, stop looting and wait a while before continuing.
If he instead finds good loot, he will go into a loot frency no piranha could have ever thought possible.
Well, if you like that, go for it. But wouldn't it be better if there just was a small chance to get some phenomenal loot at any time?
For example you could put in lines into each lootgroup with a very low chance but with a minumum of 2 times the maximum the normal would give you. Example:
In the ammo loot group there is the line
<item name="ammo762mmBulletFMJ" count="1,20"/>
Add a first line
<item name="ammo762mmBulletFMJ" count="40,80" prob="0.01"/>
Since this is the first line, every hundreth time you normally would get some small amount of ammo you would get 40-80 bullets of 7.62 instead