It should be interesting for you to note that the period correct headset for your '90 Romic (that is the headset for which your frame was designed) was a 1" threaded headset, a type for which there is an ages old standard of measurement for what is known as stack and which included both cups (portions external to headtube), both races, washer and locknut.

Further, it would have been typical to see a stack height around 40mm, depending on make/model.

So, if your Romic's head tube length is 120mm, with the original headset, the "effective stack height," as you call it, would be 160mm.

You don't clearly identify the type of integrated headset on the Giant, but an integrated headset could have very little stack, but even "zero stack" internals have some height. Lots of units, like FSA Orbits, have 8.7mm top caps, so let's call it 10mm; maybe you've got a small spacer on there. So the Giant, then, with a 150mm head tube, could have had an effective stack of 160mm as well.

So effective stacks are the same, describing equivalent minimum bar heights (for a given stem angle). Additionally, higher bar heights (for more upright posture) were certainly possible with the Romic's threaded steerer, thanks primarily to quill length height adjustment range, despite the Romic having a shorter head tube.

Of course, you do not mention other factors to influence stack, e.g. fork length or head tube angle, so for the sake of the discussion, and hopefully understanding, let's call all those equal.