alternatively you could "hack" a little bit.
set the step size to 1 and max value to the widgets width. Update the max value on every resize (and also recalculate the value).
You then create custom setter for your"real" max value and a new signal with the "real" value calculated linear to the base max value.
@raven-worx: while I'm not sure it is the right solution for what @tokafr wants to do, he can take ownership of the vertical scrollbar and have some influence this way. I have done it before that way.
@tokafr: Instead of trying to influence the hidden layout of the QTableView, you could mask the space between the header view and the vertical scrollbar to make it look like it being part of the header view. Stylesheets are probably the best place to try.