How important is it for Moltres to have 15 attack stat?

I wouldn't power a Moltres up myself. I see it as a better version of Flareon.

Attack is the most dominant factor for its CP for Moltres. Considering that you are not able to put it into gyms, I would say the attack IV is most important.

The Fast Attack loses very little dps if the Attack IV decreases to 14 and breakpoints remain similar dps at similar Pokemon levels, pretty similar for the Charged Attack numbers.

Moltres reaches its breakpoint concerning Fire Spin at level 31.5 with a 15 Attack IV... With a 14 Attack IV stat it reaches that breakpoint at level 32.0 (Only 1 additional power up).

Higher Charged Attack damage can be achieved with additional level ups beyond that point though! :D

Here you can check:
https://docs.google.com/spreadsheets/d/1u17VtfOquv2pGNskThfA_DT0PLCbSAk1Ju7lgREGjXs/edit#gid=124395849

That difference is so small in practice that no one will ever be able to notice it.

Other factors are much more important like network lag, mobile signal strength or grease on the display.