She's got a lot more going for her in terms of strength. She easily blocked an attack from a Fishman, plus Sanji was able to still use Blue Walk somewhat effectively in her body.
All depends on the situation of battle. Does Usopp have prep? Do they start out close to eachother? In a normal close range fight, Usopp would lose hard due to not having Haki or much time to come up with a plan. If Usopp starts the battle out the way he wants to, he should win when considering he was able to aid in taking down Baby 5 & Buffalo, plus it seems as though he has Kairoseki cuffs in his inventory based on him capturing Caesar.
I'm not sure if this is fake, since I haven't bothered to check the entirety of Part 2's SBS, but it seems to be a common rumour that Oda stated Part 2 Usopp's strength is on par with Part 1 Zoro's. If that's more than just a fan rumour, then Usopp stomps once he negates Croc's intangibility.
Ace (Marineford) VS Gear 3rd Luffy (Dressrosa)