Knees out doesn't appear to help pushing off my heals. Also if I attempt to keep my chest up I seem to just stick the middle of my spine in to extension.
Knees out gives you pelvis more room to sit between your heels... which in turn allows you to keep the weight further back. This is one of the amusing issues I encountered when switching to lifting shoes... you can't just squat the way you used to...
If you're slipping into extension, get your groin more mobile, or feet slightly further apart (depend on what works, naturally).
When I cue for 'chest up', I think about pushing my pelvis down and forward and levering your chest back. This might rustle jimmies on paper, but the point is to keep the thorax/abdomen rigid along a neutral spine and use the greater range afforded by the wedges in those shoes (i.e. simulated ankle range) to keep your body more upright whilst in the hole.