Just staying with A15 parts for the moment, here is what I have observed.

The factory CR rating for the 1982 A15 (the one with the H95 shroud-port head) is 8.9:1.

(Please check my numbers. I am here to learn)

Lets assume for the moment that 8.9:1 is correct. The chamber of the H95 is ~35ccs from my measurement. I measured the H89 chamber at ~31ccs. So already that's about a .5 increase in CR. Surface the head and the block (cleanup during rebuild) to remove a little more from the deck clearance and the chamber and add a .5mm or .75mm overbore (whatever piston I find) and I would be closing in on 10:1 but not over. That would be where I would like to be for use with non-premium US gas. But...

I must consider the piston dish. I measured the H95 piston offset dish at ~6ccs. I do not know what the larger diameter, shallower and centered 1980 A15 dish measures at. Does anyone know?

Discussion reference slugs:
A15 Deep Dish pistons on Ebay
A15 Shallow Dish pistons on Ebay

Edited to add: The ITM catalog PDF, page 53, states that the CR of both A15s is 8.5:1. I dunno. Just gotta measure.