This is not easy as there is no direkt comparision of instructions. The H8 can do more 16-bit, can do more bit manipulation, has MUL and DIV and is faster overall. One try:

The H8 ist faster per instruction, most need 2 states compared with 4+ on the Z80. A factor of 5 seems reasonable, this would take 10 states on a Z80 for simple instructions and 20 for more complicated ones.

A factor of 3 comes with clock speed so a H8 is 15 times faster in this calculation. If we take one doubling gives 50 to 100 Elo, this is about 200 to 400 Elo.

The Fidelity Voice Sensory Chess Challenger is given with 1394 Elo, adding the 200-400 for the faster chip this gives about 1600 to 1800 Elo compared to 1845 Elo for the Excalibur Igor. This seems reasonable.

The programs are different in size and can use varying memory 16+4 (ROM+RAM) K versus 32+1 K. Less RAM means less space for move lists so the older program cannot just be ported. I don't think a program from Nelson could be written for another processor and just a fourth of RAM with more ease than using another program or starting from scratch.