Anstruth Harp is a magic item that allows you to cast a 5th level version of cure wounds
You can use your action to play the instrument and cast one of its spells
Cure Wounds states (emphasis mine):
When you cast this spell using a spell slot of 2nd level or higher, the healing increases by 1d8 for each slot level above 1st.
Since you are not casting with a higher level spell slot, does the status of the spell as 5th level serve no benefit to the healing?