The experts have chimed in.......years ago.
In the hi-compression frankenstein thread......
Cliffnotes: EJ18 heads on a EJ25 bottom end will give you about 11.5:1 CR. Still runnable on pump gas, premium maybe....but nothing wild. Power level? not sure but likely in the mid 140's.
EJ22 heads on a EJ25 bottom end will give you about 11.1:1 CR. Power level has been posted around 150-155. I may be off on the numbers, it's been years since I read the thread.
Jay