Feature wise it's actually pretty clear. Matrox G400/G400 MAX/G450/G550 cards are DX6.1 capable. They all lack HW T&L AFAIK (the G550 apparently has one but I've never seen it exploited, akin to Savage2000) which means they are do not have feature parity with DX7, let alone DX8. The G550's extra features/capabilities were only ever exploited through very specific software such as the Headcasting demo that swaaye mentioned which I believe was intended for video conferencing. In fact, I can't think of any other application that used them. It's fair to say it is an odd chip.
The Parhelia is as much a DX9 chip as the G550 is a DX8 chip. It ain't, despite rumours and claims prior to release. As far as OpenGL goes, the G450 should have a full OpenGL ICD as the G400 cards got one in the latest drivers, so I don't see how Matrox would drop it on this card in particular.
Performance has been shown countless times by this point and you can find graphs and numbers in contemporary reviews and more modern ones such as Havli's excellent rundown for which the site seems down at the moment, unfortunately, so here's an archived link: https://web.archive.org/web/20200104235925/ht … 2011-edition-/1