Adding to my question regarding the Classic II, I just looked at a high res logic board image (since I'm too lazy to open mine) and it looks like it only has 4 RAM chips. Going by the part number shown (KM44C1000AJ- 8) it is unfortunately already using 1M x 4 ICs (which would make sense since it has 2MB(x8) onboard RAM).

Still, I wonder if it would get a speed boost using something faster than 80ns chips. The datasheets I found listed 70ns as an option for AJ, 60ns for the BJ revision, and up to 50ns for the DJ revision.

Those might also speed up the IIsi/475 as well.