RAW Metabolic Overdrive states; "Use this Stratagem in your Movement Phase, after moving a Tyranids unit from your army. You can make a second move (including Advancing, if you wish), but when you do so you must roll a dice for each model in the unit. For each roll of 1, inflict a mortal wound on the unit. The unit cannot shoot or make a charge move this turn."
The stratagem says it procs *after* you finished moving a Tyranid unit (and if you wanted to Advance you could not *finish* moving a Tyranid unit until the Advance result modified it's Move Characteristic).
The stratagem says when it procs you can make a second move *including advancing* if you wish. Nothing in the stratagem addresses any previous Advance your unit may have made.
It seems to me the stratagem allows a Tyranid unit to move a second time, including an <optional> Advance.
So is the stratagem just poorly worded?
The previously cited FAQ says 'no you can't Advance twice';
However, that
FAQ was written to clarify the rules, and did not specifically mention Metabolic Overdrive - as stratagems almost universally break/bend/create rules by their very nature (by allowing you to shoot heavy weapons after advancing, move/charge after reinforcing, add new units to your list mid-battle, etc.
So, as stratagems were designed to break rules, wouldn't what the stratagem says to do take precedence over the rules, unless an errata/
FAQ specifically mentions the stratagem in question?
EDIT: Would also like to add that where it says you cannot Advance twice, it isn't even on the topic of stratagems.