I know a minor faction has become a power before
Others have talked about this, but none of them have really pointed out that this isn't really what happened. The EG Players, a large and active player group with a faction based out of Euryale (EG Union), won the Rise to Power campaign. This let them create the lore for Yuri Grom, the leader of the EG Union. Except that Grom's Power HQ is in Clayakarma, 92Ly away.
So the minor faction did not 'become a Power'. The leaders of the player group were able to 'create a Power'.
These are vastly different things and highlights how much of a headache it would be if FDev did it again.
There was a proposal about allowing player factions to become 'freedom fighters' within a Power's structure, so they could participate in PowerPlay without having to choose an opposing Power to attack an encroaching Power.
Most Powers started with around 8 Control Systems, which is usually around 100-140 Control and Exploited Systems. Grom started with fewer, but better Control Systems: maybe 60-80 systems total?
There's an argument that can be made that a faction with that many systems under its thrall is in fact a Power, whether or not it is recognized as such.