I have a theory on ML which could easily be wrong so don't take this as bible, however it does tally up with tier 1's probably pretty well on the amount of ML i use. (i use around 20% and get roughly a 15% chance of seeing overspawns on T1's)
In short the item roll happens first, i have no idea how the additional stats would work, as ML reportedly increases the chance you get more stats on your items. This ones very hard to prove either way if it even works.
After the item roll has happened, lets suggest it was an angelic tier 1:
2 str
1 ML
2% heal mastery
2% fire mastery
A second roll will then happen based on how much ML you have, so lets say you have 30 magic luck, it has a 30% chance to occur.
This will then increase one of the stats by upto 1 tier. so
+1-5 str
+1 ML
+1-5% heal mastery
+1-2% fire mastery.
If it hits either ML/Heal mastery it would immediately be overspawn. However if it gets a low roll on either str/heal mastery, it would not. If it high rolls at 4-5 then in this example it would hit.
Similarly lets suggest a level 20 item with 10 dex drops, even the ML if it hits would only increase that to 11-15 so would remain trash.
However if it had 30 dex and then got 5 more, or in ryans case dropped with 34 then got +5 more from ML, the ML really would pay off in those instances.
This theory generally tallies up with how few of the higher tiers we get with overspawn. I don't think it will ever be proven/disproved, but it's a reasonable way to look at it.
I'm not entirely sure about multi stat items, as items with 2 overspawn stats have been seen. Unsure if i've seen one with 3 overspawn stats.
Whilst we're talking about overspawn, Lets also talk about the EE cap as this is also uncommon knowledge these days.
When paul decided to update and make some items now unspawnable, such as 200ee tier 1's (excluding suffucensing) he also decided to specifically nerf enhanced effect.
So how this works is pretty simple:
The max overspawn of an item is also the cap of ee, including prof.
There are as follows:
Tier 1: 40ee
Tier 2: 60ee
Tier 3: 80ee
Tier 4: 100ee
Tier 5: 120ee
Tier 6: 140ee
Tier 7: 160ee
Tier 8: 180ee
Tier 9: 200ee
Tier 10: 220ee
Tier 11: 240ee
Tier 12: 260ee
Tier 13: 280ee
Tier 14: 300ee
So if you go in game and you have 30 fire prof with a standard 60ee tier 3 fire, a fire with 50ee will show the exact same amount of damage in game.
However if you went to one of your mules with 0 fire prof and then compared the two fires, the 60ee one will deal 10ee worth of more damage.
Also, if you're thinking about fire mastery, all that does is simply add to your fire prof. So 60ee + 6 fire prof on the active position will be the exact same as 66 ee.
If you are using an overspawn level 5 axe with 95ee and have 20 axe prof, this also hits the cap and 80ee will deal the same on that character as the 95ee.
You can do more damage than the EE limit by using sidemods. So again presuming you have 20 axe prof and now have a choice between a 95ee lvl 5 axe or a 80ee axe with 6 dex, you'll find the second axe will deal more damage.
Stats such as int/dex/str/vitality have not been nerfed, so if you have a tier 1 with 30 dex, this still works to full value.
Life/mana per kill is the other major one which doesnt do what it says it does and is similarly capped, however this stat is rarely used these days and no longer really matters.
Just thought i'd mention it as it's long forgotten info which is becoming more relevant with more overspawn EE drops happening.
One last note is that character profiles will incorrectly show full ee damage on unspawnable ee items,
Here is an example:
damage is capped at 100ee for a level 5 charm, so the amount shown in his profile is less than he'll see in game