I'm using the s stem with aberhallo extension. There's not much flex in the m handlebar, it originates from other areas of the bike and the extra height of the m bar exacerbates the problem.

Good point, actually I don't know the thickness of that tubing, I've assumed it's similar to other sections of the bike but your experience says otherwise?
I've considered modding a stem to add height adjustability; do you have any pics online?