I'm not understimating Usopp. I'm just basing my argument on actual manga facts. I also hope for Usopp to get considerably stronger, but for now he isn't someone who can take anything what the World put against him. He didn't show anything really strong enough for now to take opponents of Wiper's level. And I also can't agree that brain always wins against muscles. Actually it's not especially in shounen. In the first part of the Grandline Usopp's role was to be a sharpshooter (and it's only dedicated to be very precicious in his shooting) and to take out all the fodder. I actually believe for now his role will be the same. Only the level of fodder changed a bit, so he had to get an upgrade.
Ussop gave pre-timeskip Luffy a good fight, and Luffy w/ out gears was about on par with Wiper. IMO, If at this point Ussop isn't able to beat Wiper then he shouldn't be going to the new world. Sure Ussop's best at taking out fodders from distance, but I don't think that means that that's all he can do.
Well, we have seen more of ussop now (although certainly not everything) and I still feel he would actually pull this off although with difficulty. Ussop certainly does not lack in firepower to begin with and he is certainly more versatile than wiper when it comes to a range fight. I don't think ussop would be unable to avoid wiper's attacks (even that beam thing) and even then his accuracy should be far superior to wiper's meaning that even at a distance it would not be strange if he gets a hit on him. Ussop has shown some ability to manipulate terrain (by growing dangerous plants) and at least 2 nukes (which should hurt wiper considerably IMO). Ussop in spite of his appearance has shown an absurd capacity to take damage, even before the timeskip when he was essentially a normal human he was able to take gargantuan amounts of damage, enough to actually somewhat take on luffy even though he was already hurt from his fight with the frankys. In this particular regard I don' think wiper can simply take out ussop in a close range fight, even if he takes a beating I would argue he should be capable of standing up and keep fighting for more than a decent while. In the end, I would give his to ussop due to his greater versatility.