How advanced could Roman technology have gotten?

Thing is, in steam engines that actually work, the steam isn't really moving objects. What's happening is that steam will gradually build up pressure, in a cylinder for instance; then cold water will call the air to rapidly condense, and the resulting change in pressure is what actually does the work. To make a steam engine like this, they would have to understand the concept of atmospheric pressure; the idea that we're all essentially swimming in a vast, invisible ocean is very counterintuitive.
I'm not sure if this is correct. Didn't Jeromino de Ayanz's steam engine (in 1606 or thereabouts) pre-date most of the research into fluid pressure?
 
If gunpowder had been known, IIRC you could get a Sten gun with Roman metallurgy.

But as RamscoopRaider says, the inefficiency of agriculture in man-hours to produce the food to feed one person is a biggie. In particular, the reliance on either slave labour (cf. Sicilian slave rebellions) and imported food (from Egypt and North Africa) mandates against seeking to vastly improve Roman agriculture through automation with draft animals, seed drills, crop rotation and the like, as it would devalue the asset holdings (i.e. field slaves) of the wealthy and the good.
What if we consider a surviving Roman Empire with a post-Constantine PoD?
Sure, the agricultural powerhouses of Egypt and Sicily would still be there, which could, in theory, hinder roman interest in developing agriculture to suit more northern climates. But could the absence of slaves help agricultural tech by a bit?
We must also consider entities outside Rome -- even if the empire itself doesn't develop the heavy plow, the germanic or slavic tribes would, if a sufficient number of them become sedentary and centralize.
 
Ignoring the OP, just responding to the thread title -- a good place to start in advancing Roman technology is to look at useful goods and/or techniques that other civilizations of the time had managed to master, of which paper-making and pig iron seem to me to be the most useful.
 
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