Is that incorrect? I guess I don't really think too much about it. Sometimes it sounds more like "Usoppuh".
Speaking of Nagapana-kun, funny that Robin calls everyone else "-San" except for Usopp, as I recall.
Not exactly incorrect, since the Japanese pronounciation according to the spelling is really "Usoppu." Blueno, for example, is still "Buruno."
Now what is it with Robin calling Usopp with a "-kun?" Is it because "Sogeki-san" doesn't fit him? Usopp rarely utilizes his sniping skills, admittedly. He is more often seen fixing the Merry or inventing stuff, so I guess Robin's kinda confused about his real position. Hence the nickname "Nagapana-kun", which Usopp doesn't seem to mind.