I built a ground effect car in 2001.
It was extremely stable at high speed. I couldn't compare it with and without the undertray though because it couldn't really be run without it.
They key is to get the centre of (minimum) pressure located in a position that is favourable to the handling you need. Too far rear and it will promote understeer, too far forward and the reverse will be true. If putting holes in the 1/12th chassis close to the front promotes high speed steering, and it IS an aerodynamic effect (big if), then this must be due to a shift in the centre of pressure under the car towards the front. But it could just be the introduction of more front flex and more mechanical grip under high loading.