In reply to HeMa: Where do you climb in Scandinavia? I live in Finland and climb most weekends in the south. I use La Sportiva Nepal Extremes which are fine down to about -15 to -20, beyond that you need something warmer - even if just out for the day. I also have Scarpa Vegas which I wore in December and January when we were climbing at -17 and they made it much more enjoyable.
I would only go for the boots you mention of something similar if where you climb it doesn't get below -10 or you only do single pitch icefalls (much easier to keep your feet warm when you can easily get back down to the ground and walk around than on multi pitch stuff). When I lived in Scotland we all climbed in plastics, but everyone did back then! Lots of Scottish climbers now use leathers, but I find it is easier to get cold feet there (standing in deep snow on some tiny ledge belay for 2 hours) then it is at -20 here in Finland, so plastic might have an advantage.
A pair of trekking shoes (approach shoes) like Merrels should be ok for via ferratta.