Besides adapting the thueros, maille and the manipular formation earlier than the greeks (which didn't bother until the galatian invasion), the Romaioi also had the willingness to send more of their people into battle. Look to Pyrrhos' invasion, the Epirote king was more than a match for the Romaioi, both in strategy, tactics and equipment. The Romaioi simply responded by literally man-spamming until they won out. I'm sure if its the Romaioi at Thermopylae, they would not even bother use the place as a bulwark; they would simply out grind the persian warmachine in manpower. Worked on Pyrrhos; worked on Hannibas; worked on the Sweboz migration. If you run out citizen soldiers? spam out the urban poor. Not enough? enfranchise the provincae populace.

if you've played as the Romaioi in EB, You'll get where I'm getting at ;)

Sorry for digressing from the topic, oops.