When I first heard Bill Barretta as Rowlf, I couldn't tell the difference. To me, it sounded pretty much like Jim's Rowlf. But after watching more scenes with Bobo and hearing Bill's Rowlf a few more times, I noticed the difference. Yeah they do sound similar and I could definitely hear Bobo in there, but as dwmckim pointed out, Jim's Rowlf does sound like Bobo at times. But there are obviously subtle differences. Plus, it's only been a couple internet videos and 1 or 2 lines here and there in some specials as far as I know, so I guess with time and more opportunities for Bill, it can develop and get better. As has been the case for some other re-casts.
As for how Bill Barretta was chosen, obviously I don't know the true story but I would think Bill performing Bobo would have something to do with it since it's the closest to Jim's vocal range for Rowlf. It does seem to me like one of the hardest voices to do and copy aswell.