Maybe it won't be a permanent power up like Zeus for Nami, but a moment in Elbaf with Usopp wielding Ragnir is very probable.
The fact is that Ragnir alone is stronger than the entire SH crew except Luffy and maybe Zoro/Sanji, so it would make Usopp too powerful for the final saga.
So there are 2 options:
1) it would be just a scene in Elbaf and then the hammer will come back to Loki
2) if he will keep it after Elbaf, in Usopp' hands Ragnir will become way weaker for some reason.