I think the evidence on turnout is pretty complicated. You're going to hearing from André Blais later this week I believe. Nobody in the world knows more about the relationship between turnout and electoral systems than André, so I would recommend you listen to him.
The reason that the citizens' assembly in B.C. chose the single transferable vote, to the surprise of everyone, was that they believed it offered them more choices, because they could choose both candidates and parties in whatever order they liked. It's true that the counting process is a more complicated one. The Irish, who use it for their national elections, don't start counting until the next day, and in some of their districts it takes two or three days to count. A few years ago in a couple of ridings they actually instituted electronic counting of the ballots, which gave them instant results, and they hated it. They actually liked the one- or two-day kind of “who's ahead on the first count, who's ahead on the second count” process and they would crowd in and watch the process. So they sold all the machines, at a huge loss, and they're now out of the electronic machine-counting business.
However, they were recognizing that there was a social dimension to the electoral process that engaged them all, and the count was part of that, as well as the vote counting.