For the rest of Europe, the worst Katla eruption in recent centuries was that of 1625, which caused catastrophic famine across northern Europe. In Scotland, this was the main impetus for the development of the coal industry - the summer was so cold and wet that peat never dried out enough to burn, adding an acute energy shortage to the misery of famine, flooding, cold and toxic smog. This seems to have caused more loss of life in Scotland than any other natural disaster since the Black Death. (And presumably the rest of northern Europe, at least - Scotland happens to be the place I know most about). Other years when it seems to have had an effect on conditions in Scotland: 1722 (following the 1721 eruption), gales causing massive crop destruction; 1755-6, famine. It may have contributed to the three-year famine of the early 1580s, but the 1580 eruption seems not to have been prolonged enough to explain all that on its own; maybe Billy Mitchell in Bougainville Island was responsible (much bigger than any Icelandic volcano). The 1755-6 eruption may have had some effect on the Seven Years War, which was ultimately what made the independence of the US possible - Britain and France beating each other senseless after a war motivated by scarcity allowed a new force to emerge.

According to this page, Katla always blows up with much the same power (within one order of magnitude):

Historical record

There's been a substantial variation in its wider effect, though.

Northern Europe always gets more affected by Icelandic eruptions than anywhere else - Britain in particular. Coming on top of the present economic crisis and peak oil in the North Sea fields, a major Katla eruption (wiping out most UK agriculture for a year or two) would put the British economy some way below where Greece is now and finally obliterate the British delusion of being a "world power".