Cleon, your reasoning is sound. I agree to the improved natural armor, as well as your feat suggestions.
Thank you kindly.
I've been thinking we may want to add a slam attack to this beastie, so if it grabs a victim with its jaws but fails to swallow them it can still attack another enemy with a body-slam or tail-slap the next round (probably trying to knock them prone with an overrun), or when it gets an attack of opportunity. After all, a Purple Worm gets two attacks, why not a Slime Worm?
If you like the idea, I'd set the damage the same as its bite. With regular damage bonuses, that works out as:
Attack: Bite +17 melee (2d6+8) or slam +17 melee (2d6+8)
Full Attack: Bite +17 melee (2d6+8) and slam +12 melee (2d6+4)
Oh, and the current conversion still has a Purple Worm's saves. I reckon they should be:
Saves: Fort +12, Ref +5, Will +2
How are we handling the hide bonus/"mimic hoard" business?
My first thought was a rewrite of the Mimic's ability Mimic Shape, something like:
Mimic Horde (Ex)
A slime worm can assume the shape and appearance of a pile of treasure that fills roughly 1000 cubic feet (e.g. a mound roughly 15 feet in diameter and 6 feet tall). The creature cannot substantially alter its size, though. A slime worm's gelatinous flesh is slimy and yielding, no matter what appearance it may present, although the treasure adhering to its hide may give it a rough and metallic texture. Anyone who examines the slime worm can detect the ruse with a successful Spot check opposed by the mimic’s Disguise check. Of course, by this time it is generally far too late.
[The 1000 cubic foot volume was set on the assumptions that a Slime Worm's flesh has the same density as a Gelatinous Cube or Gray Ooze (15 pounds per cubic foot), and an average worm weighs 15,000 pounds. Since it is the same thickness and 3/8th the length of a Purple Worm it seems reasonable to deduce it weighs 3/8th the Purple's 40,000 pounds.]
I guess you can change the Disguise to Hide, but I prefer the former since (a) it seems more appropriate and (b) it doesn't impose a size penalty.