The PS3 will only play SACD's if you replace the Model 990. It doesn't have a multichannel analog output, so they only way to hear SACD's is via HDMI. THe BDP-83 will do SACD's and DVD-Audio (which neither the BD55 nor the PS3 will do) via multichannel analog.
The Blu-ray feature sets are similar (Profile 2.0, onboard decoding of the new formats). Once you get past that, though, there are some very real differences. The BDP-83's video processor is definitely better, which will have minimal impact on Blu-ray but a clear benefit for DVD. (The BD55 has a somewhat newer and improved version of Panasonic's Uniphier chip compared to the BD30 that I have, but I was pretty underwhelmed by the BD30 for DVD performance when compared to my old 983H .) The BDP-83 supports DVD-Audio and SACD. The BDP-83 is faster (start-up, disc loading, etc.). The BDP-83 has a better analog section. The BDP-83 also benefits from OPPO's customer support, which is among the best in the industry.
I've compared a BD30 to the BDP-83, but not a BD55. The BD55 is a newer generation of the BD30 (enables onboard decoding, adds Profile 2.0, and uses what I'm told is a better version of the Uniphier chipset). If it were my $137 to spend, I'd go with the OPPO.