I thought you mean that after timeskip Usopp will be at after timeskip Sanji level^^
Sorry
Usopp is a man he is naturally stronger than Nami. And way more awesome. I mean we saw him deck Kuro even though Kuro was holding back Usopp still hit him pretty hard. He took out two bodyguards with guns. The best we've seen Nami do in east blue is take out 1 person from behind and then lose in a straight up fight against a fodder.
The very least we've never seen Usopp beaten by a fodder. People take what Oda said a little to seriously. I mean thats why Nami came to Usopp for a weapon because she was the weakest and absolutely no help in a fight.