Yes. The Polyfil makes the speaker "see" a larger airspace.

The standard for enclosures is 3/4" thick MDF. Dont use plywood. Another wood that you can use is 13 ply Baltic Birch. It cuts easier, is much lighter, and even stronger than MDF. Only problem is it costs about 3x as much, about $60 a sheet, whereas MDF is only $23 or so a sheet.

For a 2 12 or 1 12 sealed box, you should be able to get away with 1/2 a sheet of MDF.