So it turns out that we'd like to use fixed height mode, because it's faster and it doesn't constantly try to update the treeview — we saw a significant decrease in CPU use with a table that, unfortunately, may contain a few thousand rows and some 20-ish columns.
Oddly enough, merely turning off autosizing on all the columns doesn't help, one needs to set fixed height mode too.
But of course, the cell contents are of varying length (they're text and numbers), and it would be nice to update the column size time to time (ie. when I know they should be updated, and not all the time like autosizing unfortunately does).
So what I need is being able to figure out that the newly inserted row / cell has insufficient size (I guess something to do with the GtkCellRendererText and Pango will come handy), and then resize the affected GtkTreeViewColumn using set_fixed_width. I've looked at the source of GTK+ to see what they do when autosizing, but couldn't really make head or tails of it. My main problem here is getting to the text layout and/or the cell size requirements from a given TreeView/ListStore/iter combination.
I use perl-Gtk2, but answers are welcome in any commonly used language.