M2 was not on par with Dreamcast, not even close.
Texture-mapped, gouraud shaded polygon per second performance with lighting and rendering features / effects applied:
M2: 300,000 ~ 500,000
DC: 3,000,000 ~ 5,000,000
However, there were many Dreamcast games that used only a small fraction of the performance it had, so with that in mind, it's understandable that many Dreamcast games look roughly on par with games developed for M2.
With that said Daytona USA on Dreamcast is miles ahead of IMSA Racing, Daytona DC is 60fps, uses alot more polygons, much more advanced lighting (volumetric) as well as a much higher quantity of more detailed textures.
edit: The Dreamcast game Test Drive Le Mans comes to mind, is much, much more similar to IMSA Racing than DC Daytona USA.
http://uk.youtube.com/watch?v=nC2A9gErwKM
http://uk.youtube.com/watch?v=58h_tQWUTjE
http://uk.youtube.com/watch?v=4_z3hsEpzyM
Both games are 30fps. Still Dreamcast Le Mans is well beyond M2 IMSA Racing grapically, but I don't expect M2 to compete with Dreamcast anymore than I expect Dreamcast to compete with Xbox.