The Sage says that mithral heavy armor is medium armor even in this case…
The Dragon #355 Sage Advice column “Official Answers to Your Questions” includes this exchange:
Would the Heavy Armor Optimization feats… apply to heavy armors made of mithral?
Mithral heavy armor effectively becomes medium armor for all purposes, which means that benefits accruing to the wearer of the heavy armor wouldn’t apply when wearing mithral heavy armor. (79–80)
(The Sage at the time is Andy Collins.) Thus, according to the Sage, a creature wearing mithral heavy armor can't benefit from the feat Heavy Armor Optimization (Races of Stone 41).
This reader suspects that the Sage is basing this ruling partially on this exchange from the Dragon #335 Sage Advice column “Official Answers to Your Questions”:
Is a character proficient with light armor, such as a rogue, considered to be proficient with mithral breastplate? What about a character proficient with medium armor, such as a barbarian—is he considered proficient with mithral full plate armor?
The description of mithral on page 284 of the Dungeon Master’s Guide is less precise than it could be in defining how it interacts with armor proficiency rules. The simplest answer—and the one that the Sage expects most players and DMs use—is that mithral armor is treated as one category lighter for all purposes, including proficiency. This isn’t exactly what the Dungeon Master’s Guide says, but it’s a reasonable interpretation of the intent of the rule (and it’s supported by a number of precedents, including the descriptions of various specific mithral armors described on page 220 of the Dungeon Master’s Guide and a variety of NPC stat blocks).
Thus, a ranger or rogue could wear a mithral breastplate without suffering a nonproficiency penalty (since it’s treated as light armor), and each could use any ability dependent on wearing light or no armor (such as evasion or the ranger’s combat style). A barbarian could wear mithral full plate armor without suffering a nonproficiency penalty (since it’s treated as medium armor), and he could use any ability dependent on wearing medium or lighter armor (such as fast movement).
The same would be true of any other special material that uses the same or similar language as mithral (such as darkleaf, on page 120 of the Eberron Campaign Setting). (80)
Thus, for example, the Sage would have the creature receive no benefit from the feat Heavy Armor Optimization were the creature wearing mithral full plate. If such a ruling stands in your campaign, and you must still take a feat that applies to your armor, consider instead the feat Armor Specialization (Player's Handbook II 75), Efficient Defender (Cityscape 60), Guerrilla Warrior (Heroes of Battle 97–8), or Improved Armor Dance (Dragon #315 96). (However, if you were aiming for the feat Deflective Armor (RS 137), may I suggest instead the feat Parrying Shield (Lords of Madness 181) or Shield Ward (PH2 82)?)
…But you can take the Sage's advice or leave it
What authority you ascribe to the Sage's rulings is up to you. (See also this question.) Since the feat Heavy Armor Optimization is itself less than optimal, since no feat named Medium Armor Optimization exists, and since no feat offers exactly the same benefits of the feat Heavy Armor Optimization to a creature wearing medium armor, this DM would be comfortable with ignoring the Sage's advice and letting a creature that prefers to wear mithral full plate benefit from the feat Heavy Armor Optimization.