Can I use pastry flour instead of cake flour to bake a white cake?

It'll taste just fine, but the texture will be off (but probably not by much). Basically: cake flour = very low gluten; pastry flour = low gluten, AP = normal gluten, Bread flour = high gluten. The higher the gluten, the "tougher" the bread >can< be. I've never tried it, however, I would recommend trying to mix the cake batter like muffins-- add the wet to the dry at the last possible moment, and don't mix too much. The mixing can develop the gluten into long strands, making it tougher (this is part of what kneading does).

Pastry flour will not rise as much as white flour, other than that it will work. The cake will just be a bit heavier.

I would not do it.