Hi, I have the VSX-23 and like it, and my buddy has the VSX-21 and posted a
review of it last night.
I don't recall noticing much difference with the lossless codecs (e.g. Dolby TrueHD) when I upgraded my VSX-1015 to the VSX-23. I was running optical digital audio to the 1015, but like you I wanted HDMI audio (as well as SACD and DVD-A over HDMI). Some people have noted an improvement with the new lossless codecs, but it wasn't easily apparent to me. I didn't do a comparison between the two, but I didn't notice the angels singing or anything once I connected the VSX-23. There certainly was no reduction in satisfaction, though.
One huge improvement for me was being able to listen to SACD and DVD-A over HDMI.
I haven't read much about Dolby Pro Logic IIz, but I have yet to read anything that makes me think it's worth getting.