compression ratio will not only depend on pistons but the thickness of gasket used and if your cylinder head has been skimmed before or not. btw, everything that comes with this block is stock standard and should give a compresson ratio of 10.5 which is the standard figure for satria gti/putra/wira 1.8l twincam engines using 1mm gaskets. im not too sure of this figures.
also 4g91/92/93 if im not mistaken can use this block. i remember something like that. but please re-confirm with your mech!
1.5 to 1.8? definitely you will feel the power difference man! there is a saying going around that "there is no replacement for displacement"