Use a 720p preset for the comp.
It’s not the comp size, it’s the apparent layer size.
If the layer appears bigger than its original size (1:1), it will pixellate. If you enlarge it by scaling it up or by zooming in on it, it will pixellate if you go far enough.
For example: In a 720p comp, a 640×360 layer can only fill a quarter of the screen before you risk pixellation. In practice, you can make it look maybe 120% larger before you start to see pixellation. If you view the layer at a very oblique angle, the portions of the image that appear much bigger than original size will pixellate.
To see how this works, you may want to drag a copy of the source into the comp and leave it at 2D for comparison. Then you can see how much its 3D version (apparently) enlarges when you zoom in on it.
So to recap: it doesn’t matter if you zoom in or scale up a layer. If you make it *appear* much bigger than 1:1, it will pixellate.
By the way, for Illustrator layers, use the Continuous Rasterization switch. Check the Help.