I heard something similar with Jak and Daxter, where they took advantage of the embedded PS1 hardware on the PS2 to scrape together some extra performance.
Most PS2 games used the PS1 hardware for audio/sfx so you freed the main CPU for rendering/gameplay.
I believe Gran Tourismo 3 used the ps1 hardware for everything except rendering, which was the kind of nuts thing you could do if your games were single platform and with huge budget.
