Not sure what you mean by "you". if you are interested on the results, run it on the platform/in the configuration you are interested in. Before releasing OpenACS 5.8.0 we certainly run the regression tests on various configurations (linux ubuntu, rhel, mac). i would not have written "... runs on pg 9.3..." if it would not pass the regression test on openacs-core. The regression test on core shows a few errors (e.g. spell check) the same way as in OpenACS 5.6.0 or OpenACS 5.7.0.
I am not sure, why you would expect that the number of failures decreases as the pg version increases. In general, the bugs in OpenACS are not consequences of bugs in pg. I would rather expect the numbers of failures to increase, since new version of pg have properties that were unknown at the time the openacs code was developed.