Yes, adjusting torsion bars could cause a harsh ride if it's done by changing the preload. Torsion bars act as a spring by simply twisting the bar. These were first commonly used in the US by Chrysler in the late 50's early 60's.

If I remember correctly, one of the big advantages is that the spring rate is progressive which means that the further you twist it the higher the spring rate. Conversly, decreasing the preload would soften the spring rate letting the car ride lower but would allow the suspension or the shock to easily bottom out causing a hard ride on an uneven surface. This was a common problem in the old hot rod days when folks lowered cars by heating the spring with a torch. Very possibly this is what is happening in your case.

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Litton
'90 420 SEL (sold)
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