I do not think there is much doubt that the severity of punishment increases with the number of strokes. The increase in severity is most unlikely to be proportional, however.
Some report a few extra strokes makes a punishment much more severe. Others report the last few strokes are barely felt. Both are likely to be true.
Pain is entirely a subjective matter. Different individuals experience the same punishment differently. Further, nominally the same punishment, such as six-of-the-best, may in fact, involve strokes of greatly differing force / energy landing of very different targets.
The circumstances of the punishment may have a large effect. A deserved punishment with cathartic effect will cause much less distress than one considered unfair, for example. A punishment taken along with others may be easier to bear than a solo one, etc.