Okay then, I'll
Update it with those feats.
So, as far as the stats goes that leaves Challenge Rating, Advancement and Spell Resistance.
Challenge Rating
I'd eyeball it around CR 9. A common mimic advanced to 16 HD is CR 8, but it isn't as strong and doesn't get the Engulf, Crush or SR.
Advancement
A standard advancement would go like this (using the +50% for the first size stage, like the SRD Mimic).
Advancement #1 [Standard]: 16-22 HD (Huge); 23–44 HD (Gargantuan); 45-60 HD (Colossal)
I'm thinking they might progress more slowly then the usual "double per extra size" rule, since most of their increased volume is empty space - the greater mimic is not the chamber, but the walls, floor & ceiling. Maybe +66% instead of +100 per size...
Advancement #2 (2/3rd): 16–24 HD (Huge); 25–31 HD (Gargantuan); 32-45 HD (Colossal)
Hmm, I am leaning toward option #2, but I'm not 100% decided.
Spell Resistance
The monster conversion notes suggest CR+3 for 10% Magic Resistance, which converts to SR12. That seems disappointing, a 10% failure for an 11th level caster would require an SR14.
I'm tempted to either use the "direct conversion" SR 14, average the two to SR13, or give it SR15 (I'll say it's to allow for Spell Penetration, as if I need an excuse).