Hi there yes they can still get OT even while having good naps, 1) if their cumulative A time is too high and day too long and 2) if their final A to bedtime is too long.
Of course there can be many other reasons for NW - teeth, illness, SA (but not likely at her age), too hot, too cold, hunger, wet, poopy, tummy troubles...or just plain random babies-being-babies stuff :-)
If you are getting good naps then it could just be the final A time that needs to be a bit shorter. You will have to experiment as you say.
3 hr 15 is not unheard of for her age, it's just on the higher side and she may need to catch up. When we adjusted Finn's A time we often found that if we upped the first A time, the second one dropped for a bit, then when the second A time caught up, the final A was shorter. It's like they use up most of their stamina in the earlier part of the day.
Teeth often do not bother LO's during the day, they are distracted, mummy is there, etc. But when they are alone at night it can manifest - discomfort = NW.
Keep me posted.