The cylinder heads of the 790 and 865's are the same, so once the cylinders are bored and 904 pistons fitted they both have 12:1 static compression ratio.
The difference will be in the cams used by the 790 and 865 engines. The engines will have a higher dynamic compression ratio if the 865 cams are used due to the earlier closing of the inlet valves and the opposite is true of the 790 cams, i.e lower dynamic compression ratio due to the inlet valve closing later.
