The PS1 may have been a more powerful system, but the leap from the late era SNES games to PS1 is miniscule and very much comparable to the leap from the GC to the Wii. Many late era SNES games actually looked better than the majority of PS1 games. So yeah, the CD drive was the only remarkable improvement there.
Please site examples of late SNES games that looked better than PS1 games. I can't recall any to that effect.
FYI, in regards to n64 vs Dreamcast, Sonic Adventure could run rings around anything on the N64. I have never seen a game run on the N64 that looked better than the first Sonic game made on the DC.