Yeah so champ power is solely dependent on their stats, and is very unevenly weighted for stats like resistance. Some stats are valued 10 to 1 over others. 

Basically champ power doesn't mean anything about how powerful a champ actually is, and can only tell you if they're likely to be very fast (well built fast champions will often have low power) or very resistant (well built tank champs will often have higher power)

As far as mastery and gear power, the only role those factors play is their change to the champs base stats.