I'm amazed that Nami's first reaction was' why do my breasts feel lighter' and not 'the hell is wrong with my legs?!?'. I know Toei has a reputation of not considering female characters beyond a pretty face with two huge boobs attached, but I think the reaction of any normal person would've been more about the unnatural cybernetic bits and not 'oh noes flat chest :( :( :('
As for the voices thing, it's really to be expected. With one or two exceptions (like Justice League) whenever there's a body swap in an animated series they just do a voice swap. Otherwise you're asking a voice actor/actress to not only learn and play a new character, but play them through a voice they are otherwise used to playing with the original character. It's probably more demanding that it appears on the surface. And add in the fact that there are four characters involved in this, if even one of those VAs was unable to convincingly play a new character then it would've been disastrous. Sometimes a voice swap is an easy/safe choice, and sometimes it's just the only choice.
It's a shame, though, I know Kazuki Yao probably could've done a convincing Nami if he channeled his Mr. 2 mannerisms, but the others might not have been as smooth. Really, I'm just sad that this sets the precedent for FUNimation to do the same thing if they ever reach Punk Hazard. Hearing Brina Palencia act as Franky would've been amazing.