The BPD-83 has improved the RBCD sound as well compared to for example the 971 or 981.
I've tested the BDP-83 against Denon BD3800 and I actually perfer the BDP-83 for decoding the lossless over 7.1. I also tested the newer Denon A1BDci, and yes it's a little better, but with a price difference of 7-9 Oppo's for one Denon, it's an easy choice...
I'm not too certain that you will hear much of a difference at all with the Pioneer and possible it could be on a lesser level... Do you have any possibility to borrow a DV58 from a dealer and at the same time buy the Oppo? They do have a very good return policy, so this way to can actually test it out at home.